The ROM you are using has Voodoo Sound incorporated into it. You need to download the Voodoo app from the market and then use Headphone Amplifier (inside the app) to increase the volume.
I saw this in another forum...copy and pasted here:
Try this
Using dailer punch in *#*#197328640#*#*
Follow the steps
[5] AUDIO
[1] HANDSET
[1] VOLUME
[1] SRC Speech RX Volume
[5]5_lvl : 88
Menu -> Key Input
Change to any number greater than 88 (Max:100)
OK