As a preface, I run Linux for general use.
My vote goes to Exaile for playing music. Made by US swing dancers, and very robust thanks to a plugin system. I use it to fix file metadata, tag song BPMs and preview music during sets using a cheap USB sound card for output.
To manage my actual music collection I use beets, which can import, move and convert files. During file import it supports tagging through the musicbrainz database, which I also do using morituri when ripping CDs.
I'd say it's a good idea if you have fun doing it and you can use it to improve your skills. So if you want to learn more about Python and audio/video playback, you can definitely try it. It sounds like something that you can start small and keep on expanding as you learn more and more.
Audio players in Python are definitely possible, e.g. Exaile. Most of the audio processing is done lower-level in C libraries, but the user interface is done in Python. You probably want to do the same as you don't want to deal with all the audio encoding and the various formats yourself, so you need to find a library that can handle this for you. You can check out other open source players and see what they use to get an idea of the available options. Off the top of my head I can think of libsdl, libav and gstreamer. Qt can also do audio and video playback, but you only want to use it if you're using Qt anyway.
I'd say start small - a simple command line utility to play a single file. Once you got that going, improve it. Add a GUI (look into GTK+ or Qt), and improve further. Some more ideas include:
So I'd say you need to know
There are other reasons than performance. It is the fact that Python, Lua and others are not strongly typed languages, leading to harder-to-debug problems in large programs. You can avoid those by well-placed assertions and lots of unit tests though, and that is what is done when large web applications are written in these kinds of dynamic languages.
Interestingly, these days there are some quite large open source desktop applications that are written completely in a dynamic scripting language. Example: http://www.exaile.org/ (A music manager and player, in which the managing part actually also requires a bit of performance)
Mpd, I switch between window managers a lot, and prefer a music player that isn't tied to a particular frontend. I also appreciate that my music continues playing even if I have to restart X or fallback to a system console.
If you're looking for something similar to Amarok for gnome with good tagging support though, I would suggest either Exaile or Clementine.