Yes it does support mp3 files if You have the right codecs but the way I understand the OP he want’s to modify the mp3 files and as far as I know Totem does not allow for adjusting the volume in an mp3 file and saving it with the new settings.
Maybe, because as I know in KDE the codec of mp3 is given with packman repo. About Gnome I do not know how operates and if needs packman to processes mp3 files. So I ask here to learn more about Gnome.
This has nothing to do with Audacity. The user wants to be able to change the pitch of the played mp3’s. Audacity can do this. Like said, he needs lame to be installed. If you want to know more about GNOME, install it, use it, learn from the threads on GNOME in the forums.
The OP asks about Audacity which is an MP3 editor, not just a player. When you do not know what his questions are about, you should not answer and thus spoil the thread.
When you want to ask questions about Totem, please start a new thread.
When you want to know more about Gnome, install it first, try it out and when you then still have specific questions, start a thread with one question per thread and a good telling title. That is how it works here.