I have almost the same set up - Flarm = S8 = Oudie 2 and the same problem with tow planes.

I set the Oudie volume to 20% as part of my pre-flight and leave it there until I get off tow. At a comfortable time thereafter, I change the Oudie volume to 80% - which is the normal volume I like.

I find the screen touches to do that are quite easy.

That's worked decently for about 2 years now.

In another glider that had Flarm wired to an on / off switch, we just didn't turn the Flarm on until we were off of tow.

YMMV -

Lou