Welcome to Android Central! Which device? Are you saying that you're unable to access Google Assistant by saying "Ok Google" while the device is playing music?
About 2 weeks ago I broke my Pixel 2 xl and replaced it with a 3 xl. That is when the problem started. (At least I'm pretty sure it had not happened at all while I had the 2xl)
Here's what happens when I say OK Google: I hear the beep of the microphone coming on, then I say call so and so, or text whomever or create an event or whatever just like I have done literally a few thousand times before.
The problems is that the microphone immediately shuts off so it does not even start to complete my task. Although maybe 25% of the time, it does at least start to complete the task, as in "What is the name of the event?" then it shuts off.
I assumed it was a problem with the 3xl so I traded for a Note 9. It is still doing the exact same thing.
If I turn the radio off it works perfectly. I have 3 Mpow headsets that I've been using for the last year without problems so it's not the headsets.
Like I said, after I tried everything I could think of I let a tech take remote control of my phone and watched him do his thing. He tried everything, had me reboot and no luck. Then he tried everything again and nothing.
He'd never seen anything like it and I was unable to find anyone else in the world with this problem. I really need to resolve this problem. BTW, things work fine if I physically launch the Assistant mic, but that's hard to do while I'm driving.