OK, one way of getting beta testers would be posting on Reddit ... though you would need to emphasize the beta part or there might be a backlash.

Other ways on the top of my head would be writing guest posts on other music related blogs or paying them to send it to their newsletters ....

You could also cheaply? target specific YouTube videos with your ad ...

Further, if you are new to copy writing for ads, cashvertising is a great book.