In the long gone past, before i even did anything audio, i had a look at libre distros, mainly at dragora (for a couple of years).
Or just debian without enabling contrib and non-free repositories (which is not gnu-free, but free enough for me).
The main problem was if you wanted to use wireless internet, with a wired connection i had no problems.
( one user in the dragora IRC channel was interested in audio-production too, and he also is a member of the #librazik IRC channel, so perhaps he can tell you more. bill-auger, more of a fedora guy, iirc).
Looking at the output of “vrms”, which lists the packages installed from contrib or non-free, i don’t see one installed which related to audio-production by any means (for most i don’t even know when or why i installed them, and most i sure don’t need. Gotta remove them later, i guess).
So i can’t see a reason why a libre distro should give problems regarding audio-production itself.
About half a year ago i had a short look at the dyne:bolic beta iso.
Seems to do a good job, but having no dpkg/apt and not being able to install it … to me it doesn’t make much sense, but i guess they have their reasons. This doesn’t seem to relate to it being a libre distro though.
I only had a very short look, as that bugged me, so i can only say that much about it.
My uneducated claim: If you willing to give up on a little bit of comfort, like wireless (or buy a dongle which works with a free driver), it should work regarding audio (video i got no clue).
I am not sure how AV Linux and librazik and Ubuntustudio look at contrib and non-free repositories, how easy it is to just disable them and then remove those packages, with the help of “vrms”, so i guess your best bet is either try debian with just main repo (or just one of the remaining libre distros, i am not up to date anymore).
The rather unknown distribution “refracta”, based on debian in the past, now on devuan, is a good starting point for such, xfce, rather small and light, but all you need to get started, and per tradition no contrib or non-free repos enabled, most debian-based distros don’t have that, disabled contrib/non-free, as far i can tell. . Also the “dev” is in contact with the dyne:bolic dev, as far i am informed (both from devuan, duh).
The most short version: to me it usually worked good, but i don’t expect a lot from a computer.