A simple little audio player called
decibel is a python program that is incredibly fast and powerful. You do not need to import all your mp3s or other tracks before you can play them. With the full view selected from the view menu, it gives you a directory tree in the the sidebar (see the screenshot). You can expand the folder you want and either select that entire folder or just one or two tracks and then drag and drop into the main window pain (again see the screenshot). By default it is set to show just the home folder in the sidebar, but that can be changed: on the menu, go to edit > preferences > explorer and double click on file explorer.
If you go to edit > preferences there are a number of useful plugins that you can enable and you can also switch on some desktop integration features. There is also an equalizer available as well which has various presets. Again, you can keep it as a minimal player or use these other features; it is very configurable.
It is available in the repositories and more information can be found at the official site. Compared to certain other music players, which tend to be somewhat sluggish,
decibel is great and loads in seconds.
If you open
Thunderbird and go to edit > preferences > attachments you can set
decibel as the default program to open mp3 attachments.