Some Android Apps will suddenly drop audio streams shortly after going to sleep while on a wifi connection. The SomaFM App for example has this behaviour on my Nexus S under Android 4.1.
The reason is that the Power Saving Polling (PSP) mode on Android was recently changed. Apps should request a "Wifi Lock Mode" WIFI_MODE_FULL_HIGH_PERF if they want streaming to continue uninterrupted during sleep. There are at least two different Android bug reports discussing this behaviour.
A workaround is to install the free WIFI High Performance Widget and enabling it when you want to listen to a stream. It will force aforementioned Wifi Lock Mode during sleep.