Is master audio what I need?

We don't offer custom looping yet, but it is on the list of to do features. You can vote it up if you like. It's in the roadmap post in this forum. I would like to point out the obvious that you can work around this need by splitting the clip into an intro and loop section.

The other part with custom mixes of track is totally supported and we call it "Synkro". You can do it with single fully mixed tracks or actually use the "stem tracks", meaning one track for drums, one for melody, etc, and actually have dynamic mixes where different instruments or layers come in and out of the mix.

Depending on what your triggers are for changing music mix, you may be able to do it without writing any code as we have an extensive event-based script as well as integration with Playmaker and other visual scripting products. If you do have to write code, it will pretty much always be one line of basic code.

By the way, I haven't heard anything in your description that requires a pro version of Unity.

I hope that helps. Let me know if you have any other questions. We are constantly improving the product and take all suggestions seriously.