Short answer: by sending a mail to one of the main authors.

There are a lot of feature requests here.

Testing the program is a very important part of making a software. You are welcome to test the releases or the subversion code and report us bugs or requests. Even if you don't test the software but find a bug when using it, you are also welcome to report the bug.

Translations are important to many people who don't speak english or who prefer their native language. If you want to contribute with translations. poedit can be used to translate the files from transifex. The 'mp3splt-gtk' project from transifex also contains the translation for the library and the command line mp3splt client.

Libmp3splt and mp3splt-gtk need documentation. The best documentation is supposed to be the one written by the developers, but often no documentation is not written due to the lack of time.

A small mail telling the developers that the software is useful and/or that you like the software is a big contribution. Knowing that the software you create helps other people is very motivating.

If you have no time to contribute to the project but want to, you can also support it by donating.