One of the issues of UV-B5 is that it has shallow modulation when passing through some repeater systems. That is common issue with Chinese radios. R4HV posted on YouTube explanation how to fix that. As his post is in Russian language it does not popup easily on search sites.
However, modulation fix is rather simple: one resistor has to be removed and new connection made using 150 K resistor . That is all. Check his video.
Here are pictures before and after the modification:
How to fix modulation Baofeng UV-B (PVB_UV-5B-02V08)
My UV B5 cant transmit audio. Any experience to fix it?
It might be the mic connector. It contains switch that selects internal or attached microphone. That switch may fail.
Try to connect external microphone and check if it works. If it does work than it is most probable that switch failed.