Wow I love this idea, especially as someone who has spent alot of time mixing records. There are definitely some kinks to work out - as another reviewer pointed out, the loops can be of differing length which causes them to go out of phase with each other.
I also think when it comes to mixing records, the idea is to get them in sync in the first place - I was actually disappointed that you don't adjust the rotation of the records to get them to sync up, which is of course much harder to do but would be a really fresh and rewarding mechanic! In terms of what is there, I couldn't really tell which records were the right choice and how much volume to give intuitively, it seemed random.
This is a really great idea and executed well, with some polish it could genuinely be a great game. Good work!