Use AC3 filter, and set the type 3/2+SW and click the SPDIF.
Under the system, you will have to change the "restrict spdif sample rate' to both 48.1 & 44 kHz to have mp3 etc upmixed.
I have all recordings playing through 5.1 SPDIF, but leave 44 unchecked so mp3 etc play as 2 channel (the upsample doesn't sound so nice on music IMO - my amp can play 5 channel stereo which I use for music)
make sure the use AC3 encoder is checked in the system tab, as well as DTS /AC3 passthrough (if going to an amp) so it's passes through real surround instead of decoding and re-encoding.
I use system defaul as my renderer in gbpvr config.
I think I have the same on-board audio as you, and it seems to work as you want it.
"Shut up brain, or I'll stab you with a Q-Tip!"
--= Win7, C2D 2.93 GHz, ASUS 9400GT Silent, 2GB Ram, Few HDD's, 3TB unRAID server, Samsung 50" 1080p Plasma via HDMI, 40" 1080i LCD via VGA =-- * PVR2000 Analog PCI / Avermedia DVB-S PCI / Hauppauge 2200 DVB/Analog * PCH GBPVR Client * *BD-E6500 w/ NPVR client*