I have a really unusual (and frustrating) audio issue going on here...
I'm running GroovyMAME on my Lenovo W500 laptop (ATI Fire GL 5700 chipset / equivalent to HD 3650) connected to a UMSA adapter. I'm using a Hosa 3.5 mm to RCA adapter cable to route the audio from the headphone jack of my laptop to the UMSA. The onboard sound is provided by the Connxant 20561 SmartAudio HD chipset.
When I run GroovyMAME with a preset monitor spec (e.g. arcade_15), everything is fine. I hear sound coming through the connected TV. The problem is that as soon as I switch the monitor spec to "custom" no sound comes through the TV! The really weird thing is that if I pull the 3.5 mm plug from my laptop I can hear the sound coming through the laptop speakers. If I use a preset monitor spec I can hear the sound through the TV or laptop speakers, according to whether my audio cable is plugged in.
I find this really strange and don't know what's going on.

Obviously I want to have the sound routed through the TV's speaker but I'd also like to fine tune the picture using a custom setup.
Any clues as to what's going on?