Voicemail records zero size files on only one mailbox. Others are OK!

I have a weird problem on ONE mailbox - 5561.
When recording a message, it creates a tmp file as normal.
But the .al file is 0k and the wav file is 44 bytes.
If I set the recordings as al|wav, the console shows the the file was zero bytes.
If I set the recordings as wav only, it doesn’t complain, but of course, it’s a blank file.

This is REALLY weird, and I can’t figure it out as all the permissions on all the voicemail folders are correct.

Here’s the log

 -- <PJSIP/6001-00000001> Playing 'press-1.alaw' (language 'en_GB')
    -- <PJSIP/6001-00000001> Playing 'to-leave-message-for.alaw' (language 'en_GB')
    -- Executing [1@menu9:1] VoiceMail("PJSIP/6001-00000001", "5561@default,u") in new stack
    -- <PJSIP/6001-00000001> Playing '/var/spool/asterisk/voicemail/default/5561/unavail.alaw' (language 'en_GB')
    -- <PJSIP/6001-00000001> Playing 'vm-intro.alaw' (language 'en_GB')
    -- <PJSIP/6001-00000001> Playing 'beep.alaw' (language 'en_GB')
    -- Recording the message
    -- x=0, open writing:  /var/spool/asterisk/voicemail/default/5561/tmp/3dTeSn format: wav, 0x7fc300004a48
    -- User ended message by pressing #
    -- <PJSIP/6001-00000001> Playing 'auth-thankyou.alaw' (language 'en_GB')
    -- <PJSIP/6001-00000001> Playing 'vm-review.alaw' (language 'en_GB')
    -- Reviewing the message
    -- <PJSIP/6001-00000001> Playing '/var/spool/asterisk/voicemail/default/5561/tmp/3dTeSn.slin' (language 'en_GB')
    -- <PJSIP/6001-00000001> Playing 'vm-goodbye.alaw' (language 'en_GB')

But of course, 3dTeSn.slin doesn’t exist, and 3dTeSn.wav is only 44 bytes, no matter how long I record for.

And here’s my (modified!) voicemail.conf

5551 => 1234,Name,test@example.com
5552 => 1234,Name,test@example.com
5561 => 1234,Name,test@example.com
5571 => 1234,Name,test@example.com

I’m an idiot - I had
same => n,Set(MUTEAUDIO(in)=on)
and of course, this meant no audio to record, hence the zero size file. Ugh!