tweettweet51 i had this temporary drop in volume than back again problem, i run hard wired from headphone out on my ipad, it took me a lot of stuffing around to eventually fix it.
I knew what my cause was but i could not fix it on my own, as Peter said notifications triggered by my phone would effect my iPad as well due to FOCUS sharing with other devices , even though i had notifications turned off on my ipad the phone notifications still had an effect on my ipad while performing , i had aeroplane mode turned on my ipad as well to make sure but still i had interruptions, eventually with Apple support help and lots of stuffing around with stupid, unnecessary, complex, fiddling with FOCUS and still not fully understanding to why this was happening the problem went away.
So my suggestion is look at focus settings and disable everything in sight and see if the problem disappears.
Good luck! i keep telling apple keep it simple stupid , but they wont listen 😂