The display works, so the MCU is likely alive. But audio? The manual’s schematic shows that the audio path from the speaker amplifier (IC6) is separate from the MCU. Inject a 1kHz tone at the volume IC input; if you hear it on a scope but not the speaker, the amplifier IC or speaker wire is bad.
While a formal service manual (containing schematics and component-level repair data) for the QYT KT-7900D qyt kt7900d service manual
"We’re closed," the man said without looking up. He was holding a pair of tweezers, positioning a surface-mount resistor smaller than a grain of rice. The display works, so the MCU is likely alive