A few extra tips for those out to collaborate
As it is pointed out in the article "All collaborations remain open for alternative remixes", you are free to choose any backing track and to post a remix.
Still, here are a few hints which you may find helpful when picking a collaboration to join:
Keep an eye on the upload date of tracks
As you may imagine, there are tracks which have been uploaded several years ago in our library (wikiloops started 2011), and there is absolutely nothing wrong with digging up such old tracks and refreshing them with a new remix.
The only downside to such "aged" tracks is that the musician who shared them may not be frequently active on wikiloops any longer, so if you are out after feedback from the musicians you are joining, you might have a better chance to achieve that when joining recently posted tracks.
Keep an eye at the number of remixes of a track
Again, you are free to do as you like, but if you are out to get some excited reactions to your remix, you may have better chances when appearing as the first musician who added something
then when appearing as the 124th bass player who had a go at a heavily remixed template.
If you are about to get started on wikiloops,
you may also like to have a look at the tips to increase your chance to get heard & remixed.