Can we have HTPC config? as some motherborads have SPDIF/Digital output -In some cases you need to unable it or you require special motherboard cables or output cables -
If this is unabled you can digitally output Dolby Digital or DTS and your AVR can decode it
Or as spiro says Decode it at pc level and output as analouge and give 6 inanalougue inputs to your avar
Also if your motherboard / soundcard is capable of decoding Dolby true hd ,of DTS master HD and you are planning to play Blue Ray or compatible rips playing this uncompressed ,hi resolution formats you need to decode these formats on PC levels only ( if possible ) and then give 5.1 analougue out toi your AVR as your avr is NOT capable of decoding Dolby true hd ,of DTS master HD...
Pls give us config motherborad and sound card model no with weblinks