Playing and normalizing .avi and others for 5.1

[Russ Taylor]

I have what I know is an antiquated set-up. I have hardwired an older computer to both an HD TV and a 5.1 receiver. I am sure these same troubles have been experienced and addressed by others but I am not sure how to search for it.
Audio, both channel separation and syncing is my main issue but video pix elation is also a small problem. Having said that is there someway normalize the audio channels so it would make my viewing (and listening)experience better. Of course there is usually a great audio difference depending on which player i use. Should I try and convert the files to a single file type and if so which one should I use in order to try and get 5.1 sound working as well as possible?

I have a Envy24 Family Audio(VDIM) card using optical audio out to my JVC 5.1 receiver, if that helps.
Thanks in advance for any help.
Russ