1. the /var/lib/asterisk/moh is missing (or has the wrong permissions). Be sure it’s there and has the right owner (asterix:asterix) and permissions
2. Sox has to be installed. What the error logs miss to inform at the first glance is that you also need mpg123 otherwise you’ll get this:
Error Processing: “sox failed to convert file and original could not be copied as a fall back” for Johnny Cash – Solitary Man.mp3!
This is not a fatal error, your Music on Hold may still work.