Thanks for playing! I do plan to add tooltips and such in an update soon but I've updated the game description to be clearer about the controls in general.
There is a single music track that loops consistently throughout the game - it's worked whenever I or any of my friends have tested it :[ Are you perhaps using the Mac version? I don't have a Mac so I haven't been able to test that, although it should be the same as the Windows version.
You're certainly right about the gameplay being repetitive... This game was quite inspired by a game called Vampire Survivors which has a similar concept but is executed a lot more smoothly and is faster paced. I do recommend since it really does the concept justice. This game has been quite the learning experience in that regard... The active ability cards were an attempt to spice the gameplay up but it does seem like they need a buff. And if you found the first level to be long... a full playthrough through all the levels takes about 60 minutes, eheheh. Although I like to think it gets more exciting beyond the first level once your cards get stronger and such.
Also, I double checked and it turns out that the movement speed stat upgrade does work, it was just such a tiny amount that the results weren't really visible. I gave it a pretty substantial buff and it seems to be a noticeable change now.
That bug with enemies being left over was super frustrating as I couldn't reproduce it in the Unity editor, but I've since solved it - the latest version should be fully playable.
Thanks for the feedback! :)