Is there a way to change the audio level of the microphone while recording.
I go to a lot of concerts and the sound is terrible.

There is!
Go to the Settings menu and adjust it there. The default is "30".
The built-in mic is terrible, you'd get much better results using something like a Rode VideoMic.
(The above is for the Osmo+, you didn't say which Osmo you own.)

I'm using an Osmo Mobile

Hi, Sir, sorry that Osmo Mobile records audio via the microphone in your mobile phone, I'm afraid there is no other way to adjust its level, at least no way in DJI Go.