you will want to check if your system can accept 5.1 via optical or coax (will be compressed sound, but it may or may not support this).
if it does, you'd need a soundcard which has optical or coax out that also supports ddl or another format for compressing audio to optical/coax.
often cheap home theater systems are very limited on what they support for inputs. it is not uncommon for a 5.1 system to only accept 2.0 stereo as input which means even if you get optical/coax it may not work how you want.