I understand your concerns, but the words "non-exclusive right" means that the only rights signed away are your right to disallow Cafesaxophone to use the content in a broad context.
You still retain all your copyright.
I admit it appears to imply more than would actually be fair or realistic (such is the problem of trying to word something as one paragraph that would normally be a 20 page contract)
The reasoning behind this is that (for example) we do not want people to post a video, initiate a lot of discussion in a thread about that track, and then make a nonsense of the whole thread by demanding its removal, as it is the main or only source of that discussion.
The wording about "reproduce, modify, adapt, publish, translate, create derivative works from, disclose, post, remove and/or distribute such materials or incorporate such materials into any form, medium," is there to cover automatic use outside of the actual forum, for example i.e. pushing the content through a social feed, that feed may not use the original authors name, and may in fact use the forum's name.
Because we have no control over whether the social medium in this case may adapt, or lack an attribution, means this clause would stop the scenario of someone saying " hey I posted that video on the forum, I didn't grant you the right to then put it on Facebook"
Of course I understand the wording "distribute such materials" may be worrying if it implies we have the right to sell them on, and that is something I should probably look into. Again, it's down to trying to make it concise rather than over lengthy. I could include the word "non-commercially" to preclude that, however again it could be argued that everything is commercial if on a medium that is monetised, ie with advertising revenue on the same page.