Done some more testing now. First audio, I've tried aac, opus, ac3, vorbis, dts, mp3, alac and flac and I haven't had a single glitch with audio so far. And hopefully it stays that way.

Now for video, I've focused on three different formats, h264, xvid and vp8. H264 just seems to work and I haven't noticed any problems so far. With vp8 and xvid on the other hand there are problems with some files. If I take a legal example like BBB:

This one works:
http://video.webmfiles.org/big-buck-bunny_trailer.webm

but this one doesn't:
http://bennugd-vlc.googlecode.com/fi...unny_480p.webm

And for xvid there's this one that doesn't work either:
http://thepiratebay.se/torrent/53160...Rip.XviD-MEDiC

And for other mpeg4 codecs this one works:
http://download.blender.org/peach/bi...rround-fix.avi

but this one doesn't:
http://download.blender.org/peach/bi...80p_stereo.avi

So to sum it up, there's something missing in the gstreamer pipeline for some files, possibly just a ffmpegcolorspace element. IIRC the jolla gstreamer dev has posted on TMO and if you want help with testing stuff but don't want to do it in the open due to legal reason you can pm me and we can take it from there. This bug is probably in opensource code but to find it in the huge qt codebase is not that easy if you don't know where to look.