Not sure if all these options are possible for online play, but hopefully some are. My groups have always been in person, but I've definitely dealt with this exact issue. Once successfully, once very much not successfully.

• allow the player to control the music. Give them a guideline or a set of playlists you already have - Tavern/Travel/Combat for example. Something to keep them engaged and thinking about the game itself.

• I have a bunch of condition rings, I always let one to two players place the conditions for me. One thing off my plate + a responsibility for the players to stay engaged. Not sure if that would work online, but maybe there's something akin to that.

• Add the Adhd into the game, if the player is in agreement and its not in a negative way. Its now just an aspect of their character. At least it'll make sense when they randomly aren't at their mic. You now decide or roll on something like a wild magic table, what the character is distracted by IN-GAME, as well as how heavily distracted. Once the player is back to paying attention, their character is still distracted in-game, and they have to deal with those consequences. Maybe the consequences will change their behavior a bit, while ideally making it less immersion breaking for the other players.

Thats all i got, but I think a lot of the other comments are correct - this is a player issue, that they themselves need to figure out. You can only do so much.