I guess, thank you for responses...
But I think I was pretty clear on the communication about upper 10% (most popular!) requests of this section of the forum.

Beta testing is a very broad term and does not apply directly to this request. More like a "tester on demand" for specific feature.(Uber tester?)

So, for example user says:
"I wish to have...more tracks in the mixer"
20 users respond positively to the request and possibly offering some variants of same core request...

Developer steps in and says: yes, we are working on it, but we need "x" volunteers to find bugs and test this particular feature on different machines. To participate please reply to email oooo@ooo.com with subject: Thread#5551212


To summarize, the beta-tester could be a random person who is specifically and genuinly interested in a particular feature and is willing to dedicate some time in testing it.