When running MPD, the Music Player Daemon, you can run into the error

listen: bind to 127.0.0.1:6600 failed: Address already in use (continuing anyway, because at least one address is bound)

Searching in search engines yields many questions, but no answers that I could find. The key issue seems to be in the setting

bind_to_address “any”

This would make it listen on all ip addresses and protocols. However, it only will listen on ipv6, not any ipv4 address.
Changing this setting to an ipv4 address does make it listen on all ipv4 addresses

bind_to_address “0.0.0.0”