CD changer works but quietly

Just tried to install an Alpine CHM-S630 changer to a Symphony HU via an interface.

The HU controls the CD changer (changes discs, skips tracks etc.) and allows the CD to play at normal volume for a second before becoming silent.

If you turn the volume up full on the HU you can just about hear the CD playing but the sound is muffled and bass heavy. If you skip a track it plays normally again for a second and then goes quiet.

Any help would be appreciated.

Cheers