I can confirm you I am having some problems with sound too. I don't know if it is the same bug:
- Samsung earphones connected to Mi 9SE via USB-C adapter
- Sound (spotify, tunein,...) works fine -> "Double tap to wake up screen" -> Sound volume decreases to zero (randomly) -> Disconnect & reconnect MI9 SE - USB-C adapter -> Sound works fine
- It happens in both stock Global version and xiaomi.eu weekly roms. I already sent a ticket via Mi Community APP with no answer.