We’re experiencing the same issue, which only started occurring on iOS 26 and later. We can consistently reproduce it on some devices, but not others. Once it occurs, restarting the device is the only way we’ve found to restore audio.

You can check your iTMSTransporter position is in Transporter.app or download from Apple webview. I've downloaded iTMSTransporter from https://help.apple.com/itc/transporteruserguide/en.lproj/static.html#apdAe41970bd and change my command line from /Applications/Transporter.app/Contents/itms/bin/iTMSTransporter to /usr/local/itms/bin/iTMSTransporter to solve this problem.