Seems to be resetting a few things. Try creating a value in the value tab for it, creating a new automation, setting the source to the value you just created, the target to scalesource 1, & the type to 100:1.
Seems to be working for me. Ignore most of the stuff in the screenshot. I have been messing about setting up a dynamic footstep system for the player character. I've managed to wrap my head around the sends & automations. Kind of got a switch container to work too, but it would randomly play a loud sound every so often instead of the sound I specified - probably a bug - so I scrapped that off for now & went back to the play sound action part.
* edit: did you actually assign the echo mixer as the new music sound channel? You have to assign sounds to it or overwrite the default channels for it to actually work. Also meant set type as 100:1. I think that's the one that converts 100 into a decimal value by sharing it by 100 - or maybe it's the 1:100 that does that, I'm not sure.