it is hard to give a general or either an exact step-by-step guide regarding the audio driver installation in PE
i load the driver manually
all i can say what i did on my Intel P965\ICH8 based machine after my PE booted:
1. via 'Device Manager' i update my 'SMBus Controller'
pointing to the folder where i extracted my chipset driver package: infinst911autol.zip,
for my Intel chipset i downloaded the 'Intel Chipset Software Installation Utility'
i.e. the 'INF Update Utility 126.96.36.1999' from here
2. load the audio card's driver manually in the running PE
i have AnalogDevices 'SoundMAX Integrated Digital HD Audio'
its driver package for Vista seems can work also in Win7 based PE
at build time i place the driver files extracted from the driver package supplied by the vendor into an unique folder
and point to this folder using the filebox in my script interface
so all my audio driver files are located in my 'windows\inf' folder next to the files:
either in command line i use
(in my audio driver package for Vista there is an .inf file called 'ADIHdAud.inf')
either via 'Device Manager' i update the audio card's driver pointing to this 'ADIHdAud.inf',
in Device Manager it is listed as an 'Unknown device',
but looking at Properties > Details and based on the 'Hardware Ids' i suppose it might be the sound card
and i select 'Update Driver Software...'
3. finally i use the tool 'X:\Program Files\AudioActivatorPE\AudioActivatorPE.exe'
this last step is mandatory