Microphone in use

first, what's your device model? and is the device up to date?
second, do you have some other app running in the background?
you could try to see what app is hogging the mic, the message itself I don't think is an issue... rather the fact that something else that you are not aware of is using the mic seems to be the issue.
lastly, If you don't want to bother tracking it down. you can submit an issue report on Samsung Members app after trying to wake up the assistant again. It'll collect log of what the device was doing at the time and hopefully find what was using the mic at that moment.
 
You'll see messages like this when something is using whatever you're trying to use. I've seen more of these type issues with the camera than with the mic but it is pretty simple to track down in this case. Go into Settings>Apps>three dots in upper right corner Permission manager>Microphone. There you'll see a list of apps that have permission to use your mic. Take permission away from all of them. This doesn't permanently change anything. The next time you run one of those apps it'll act like it's the first time and ask for that permission again.

Afterwards you'll want to keep track of which apps ask for permission for a while that way if the issue returns you can say, Microphone Studio (generic name) just requested that permission and now I'm having that issue again, giving you a place to start looking for the problem. If you're lucky the problem app either won't ever request that permission again or won't cause the problem the next time it does because it was just a bug.

Not sure how long it would take to a response from Samsung but Samsung Members is a joke, if you'd ever even get an answer. The mods there usually have no clue about simple Android issues even when it relates to Samsung devices. I've seen so many times that they reply with scripted responses that are not related to the problem or ones that the user has clearly stated they've already tried. Honestly I would trust Bing to give me the number for 911 before I would trust anything that comes from Samsung Members and you know Bing 99% apt to get it wrong.
 
Try bringing up the App Overview when that happens and scroll through them -- anything there that might be more likely to be using the mic?
 

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