Setting Audio Bitrate in TMPGEnc

[bluevine]

I am trying to create an SVCD. I do this by frameserving my AVI files in VirtualDub into TMPG and create the MPEG2 file. I'm working with a long video and want to fit it all on one 80 minute CD so, I need to change the video bitrate and move it lower than the standard of 2520kbps. I know how to do this no problem. But in order to maximize the video bitrate, I'd like to lower the audio bitrate somewhat.
However, when I go into the Audio tab in TMPG, the options to change the audio bit rate are "Grayed out" (in other words, it won't let me select a different rate other than the 224Kbps). Is there some other setting in TMPG that I have to monkey with in order to lower the audio bitrate to 128 or thereabouts? Do I instead change something in VirtualDub in the frameserve process?
Many thanks

[Treebeard]

You have to click the load button, select extras, then select unlock.mcf]

this will unlock the audio so you can change it

[Digifreak]

"Load" the "Unlock" template in the "Extra" folder, this will allow you top change the greyed out areas.

[bluevine]

Great - thanks