Background: I have no personal experience with the Retevis Ra-25, however I do have much experience with the Anytone AT-779UV and the Radioddity DB20-G, both of which are clones of the Ra-25, with the possible exception of how CPS software is implemented and which is not relevant to your question.
I know of no way to adjust the microphone gain on these radios.
However, some have suggested that the small mic hole above the 2 & 3 keys on the Mic may not be in proper alignment with the internal microphone and that by opening up the case, this can be adjusted.
Since I have no reports of low audio on either of my two of these radios, I will not be opening the microphone case at any time soon.
But if you are so inclined to do so, let us know how it works out for you.
NOTE: Perhaps this is a secret feature of the Ra-25 and not the other two. If that is the case and you discover a solution again, please let us know.