I checked and it is pretty ordinary windows home 32bit. Also I read before, that firefox relies on OS to play mp3 files too, but it somehow works there.
I've installed other chromium based browser - Vivaldi and it has the same problem. I think the problem may be connected to my laptop being atom based netbook or to chromium itself.
 To avoid patent issues, support for MP3 is not built directly into Firefox. Instead it relies on support from the OS. Firefox supports this format on the following platforms: Windows Vista+ since Firefox 22.0, Android since Firefox 20.0, Firefox OS since Firefox 15.0, Linux since Firefox 26.0 (relies on GStreamer codecs) and OS X 10.7 since Firefox 35.0.