So, webm channels could have any names, huh. You always used main_gfx_attacks, I figured it's a must have.
Yeah, any name but the thing is that renpy can play only one track per channel properly (unless it's "audio" channel, which is special and can be used to play any amount of sound tracks simultaneously). So main_gfx_channel is fine as long as it's the main gfx attack which we seem to never run more that one of. If you're making gfx for sprite damage or defense, you can make up channel names like "defence", "you_shall_not_pass", "OUCH" and etc.

If we play movies at the same time but on different channels, it should work much better.
Edit: Ren'Py will keep the channels, I don't think it gets rid of them over time. They are really light weight but you might want to standardize their name somehow. If you have some effects that gotta be played "per target" like damage or main gfx animations and they don't work, we'll have to define them three times on a channel with default name + target index. I'll make sure that it is anticipated in code but this may not be required (so it needs testing).