Collaborate in audio
Soundtrap is great for when you're collaborating on the same soundfile online. You can record with your mic right into the space, or upload MP3 files, or use its loop library to create sounds. Once you're finished you could download your file as an MP3 and possibly share in Soundcloud. But, you could also share out a public link from Soundtrap itself.
How to find free music for videos lists 33 free music resources, and it also explains the types of licenses for music use.
Free Music Archive is a platform for collaboration between curators and artists, including radio stations, netlabels, venues, artist collectives, museums, music festivals and more. The FMA provides a legal and technological framework for curators, artists, and listeners to harness the potential of music sharing.
Free Music Archive on YouTube.
Royalty-free music on Incompetech