EDIT: Found the solution in this wiki page under the heading 'Selecting the Soundcard'http://sourceforge.net/apps/mediawiki/subsonic/index.php?title=Players#JukeboxKinda bumping this cause I have the same problem. It's funny, cause it works sometimes, and I can't seem to figure out what is changing.
I've tried using the Android app, and DSub. I've had this problem running Ubuntu 12.04 and Linux Mint. If it helps, here's something suspicious looking from my subsonic.log
[2013-04-27 20:21:26,488] ERROR JukeboxService - Error in jukebox: javax.sound.sampled.LineUnavailableException
javax.sound.sampled.LineUnavailableException
at org.classpath.icedtea.pulseaudio.PulseAudioMixer.openImpl(PulseAudioMixer.java:714)
at org.classpath.icedtea.pulseaudio.PulseAudioMixer.openLocal(PulseAudioMixer.java:588)
at org.classpath.icedtea.pulseaudio.PulseAudioMixer.openLocal(PulseAudioMixer.java:584)
at org.classpath.icedtea.pulseaudio.PulseAudioMixer.open(PulseAudioMixer.java:579)
at org.classpath.icedtea.pulseaudio.PulseAudioDataLine.open(PulseAudioDataLine.java:94)
at org.classpath.icedtea.pulseaudio.PulseAudioSourceDataLine.open(PulseAudioSourceDataLine.java:75)
at org.classpath.icedtea.pulseaudio.PulseAudioSourceDataLine.open(PulseAudioSourceDataLine.java:90)
at net.sourceforge.subsonic.service.jukebox.AudioPlayer.<init>(AudioPlayer.java:61)
at net.sourceforge.subsonic.service.JukeboxService.play(JukeboxService.java:112)
at net.sourceforge.subsonic.service.JukeboxService.updateJukebox(JukeboxService.java:80)
at net.sourceforge.subsonic.ajax.PlayQueueService.convert(PlayQueueService.java:361)
at net.sourceforge.subsonic.ajax.PlayQueueService.doSkip(PlayQueueService.java:108)
at net.sourceforge.subsonic.controller.RESTController.jukeboxControl(RESTController.java:598)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:616)
at org.springframework.web.servlet.mvc.multiaction.MultiActionController.invokeNamedMethod(MultiActionController.java:473)
at org.springframework.web.servlet.mvc.multiaction.MultiActionController.handleRequestInternal(MultiActionController.java:410)
at org.springframework.web.servlet.mvc.AbstractController.handleRequest(AbstractController.java:153)
at org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter.handle(SimpleControllerHandlerAdapter.java:48)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:875)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:807)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:571)
at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:511)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1093)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:265)
at org.acegisecurity.intercept.web.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:107)
at org.acegisecurity.intercept.web.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:72)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
at org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:166)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
at org.acegisecurity.wrapper.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:81)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
at net.sourceforge.subsonic.security.RESTRequestParameterProcessingFilter.doFilter(RESTRequestParameterProcessingFilter.java:110)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
at org.acegisecurity.ui.basicauth.BasicProcessingFilter.doFilter(BasicProcessingFilter.java:173)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
at org.acegisecurity.ui.logout.LogoutFilter.doFilter(LogoutFilter.java:110)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
at org.acegisecurity.util.FilterChainProxy.doFilter(FilterChainProxy.java:149)
at org.acegisecurity.util.FilterToBeanProxy.doFilter(FilterToBeanProxy.java:98)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
at net.sourceforge.subsonic.filter.RequestEncodingFilter.doFilter(RequestEncodingFilter.java:43)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
at net.sourceforge.subsonic.filter.RESTFilter.doFilter(RESTFilter.java:51)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
at net.sourceforge.subsonic.filter.ParameterDecodingFilter.doFilter(ParameterDecodingFilter.java:54)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
at net.sourceforge.subsonic.filter.BootstrapVerificationFilter.doFilter(BootstrapVerificationFilter.java:54)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:360)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:712)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:139)
at org.mortbay.jetty.Server.handle(Server.java:313)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:506)
at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:830)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:514)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:381)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:396)
at org.mortbay.thread.BoundedThreadPool$PoolThread.run(BoundedThreadPool.java:442)
[2013-04-27 20:21:26,497] DEBUG InputStreamReaderThread - (/var/subsonic/transcode/ffmpeg) av_interleaved_write_frame(): Operation not permitted
[2013-04-27 20:21:26,497] DEBUG InputStreamReaderThread - (/var/subsonic/transcode/ffmpeg) Received signal 15: terminating.