This is a short list of recommended websites. The links open in a new window.
Metronaut (by Antescofo) - A new app which enables you to play along to accompaniments while watching the sheet music. It also allows you to modify the tempos, and even to adapt to your playing.
MP3 Accompanist - this website contains accompaniment tracks to many vocal works, including operatic arias and lieder.
Pianotracks for Musicals - this website offers a fine selection of backing tracks for numbers from famous musicals.
IMSLP - the best place on the internet to find public domain sheet music. An outstanding selection of over 75,000 works.
Violin Lab - teaching, guiding, and inspiring violinists of all levels through online violin video tutorials, community support, and feedback from experts.
Amazing Slow Downer® - for sound quality, I consider this the best software to slow down or speed up audio tracks. Up to +/- 15% tempo changes can be employed with virtually no sound degredation, and even beyond this results are very good. The desktop (PC and Mac) software is good value at $49 plus taxes, but the mobile app is a good alternative and cheaper still. You can download a trial version of the software for free.
Audacity® - a free (open-source) wave editor which enables you to edit the tracks on this website to some extent. You can add or reduce periods of silence, mute or add click-beats and splice together loops or repeats. You can also change the pitch and tempos, either for the whole track or just parts of it.