I am using Vloume+ so I can hear my phone (slight dig at GNex...LOL). In order for Volume+ to work properly I have to disable Music FX. When Music FX is disabled and I chose <settings>, <sound>, <Music Effects> the Settings Application force closes. If I enable Music FX and go into those same settings I get the option to chose Music FX or Volume+ as the default app. When it is disabled the settings crashes every time I go into that option.
I did not attempt to disable Music FX before installing the GummyNex Rom so I do not know if this is an issue with ICS or the stock ROM that came with the Nexus. And Volume+ seems to work just fine even though the settings crashes when MusicFX is disabled. Just reporting my observations for the development.