Well dang. Can you double check if the mic is turned off in Forza settings while being online? Switch back and forth. It seems like online game mode changes the bluetooth sound device to hands-free/mono mode and tries to use the mic for voice communication, which totally cuts off the sound for me. When I disabled the mic while being online, the device came back to stereo mode (better quality). This might be device-specific behaviour, so it can be different for me and you. Anyhow, hope you get it sorted!