I am using Audacity 2.0.5, installed by the Ubuntu package.
I open any mp3 file and then play it using output device HDA Intel: VT2020 Analog (hw: 0,0).
The sound seems to be at the right speed but is a bit distorted.
However, if I play the mp3 with mplayer, it sounds right
Are you using too high a project rate? Sometimes 48000 Hz is best on Linux. Are your Audacity tracks at the same rate as the project rate? Are you using too low a buffer setting in Recording Preferences? That setting affects playback too.
Anything that works is reasonable if it has no bad consequences elsewhere. You don’t need to worry about Audio to buffer being higher as long as you are not doing overdub recordings in Audacity or as long as you aren’t streaming the Audacity recording to your SIP client for your show.
If you had it set to something unreasonable like 20 before, no it’s not a bug. We have not heard the problem for ourselves so we don’t know what it is.
The “Audio to buffer” setting is not likely to be relevant to that unless you are importing a large number of tracks stacked on top of each other.
If you mean you want the files to play mixed together, that happens anyway. If you are trying to join the files end-to-end, see Audacity Manual. If you are joining the files and no other editing, you should not even do it in Audacity because you will have the quality loss of re-encoding the file. Use something like Mp3Wrap instead, which joins MP3 files losslessly.