Update on previous post:
I installed gstreamer mp3 codec from software centre and it made no difference to mp3.
Then I installed ogg converter and converted an mp3 file to ogg and then the ogg file played perfectly.
So what are the difficulties with playing mp3 caused by?
I just installed Ubuntu Mate from the image I downloaded onto a Raspberry Pi 2. Mostly it seems okay, but sometimes a bit slow, including in Firefox, but it is pretty awful in YouTube, or when playing an audio file in Audacious, Rhythmbox or VLC. It plays very stuttery in the audio apps, and there i...