Welcome to Android Central. You'll need to give us a bit more information before we can really offer you any advice. To reply to this thread please create an account. This link will help you do that...
https://forums.androidcentral.com/as...-new-post.html
What phone do you have?
What OS are you running? I ask this because when Pie first came out this was a common complaint. I think it was a bug and factory reset was the answer... Perhaps someone else will chime in and offer a bit more if they remember.
Are you using a Launcher other than the stock Launcher? Anything like Nova Launcher...
You mentioned that you seem to notice this most when connected to other devices. Often phones have a built-in setting that reduces the volume when connecting to other devices as so not to cause hearing damage by being overly loud. You'll probably notice this more with headphones and Bluetooth devices. Some devices will have settings for headphones and individual Bluetooth devices that can be adjusted. I mention this because, and I really don't know in your situation but, if your device reduces volume when connected to your TV and instead of increasing the device volume you're turning up your TV to get the sound you want, the volume may stay at the lower level even after disconnecting. Resulting in the issue you're having.
Get back to us with some details and we'll do what we can to help you.
Best wishes.
