Here's the FAQ on DD5.1 audio. Perhaps this will help you sort out your issue.
As mentioned above, you need a digital audio connection and you also need to set the DVD player properly to bitstream, not PCM, etc. I assume you're talking about a HTiB system? What's the model number? If it is an HTiB, then there is no connection, however, sometimes people invoke the incorrect surround sound mode in the HTiB itself. The AVR/HTiB systems often have many different surround sound modes, with there typically only being one "correct" mode for each type of input format. For example many people invoke "5Ch stereo" or something similar and that mode should really only be used for music.
Also, on the DVD itself, there are often several different audio tracks. You need to select the correct one at the menu for the disc - usually under audio options. Some disks may default to DD2.0 or may have DTS on them instead of DD5.1, etc. Often you can cycle through the various audio tracks by repeatedly pressing a button on the remote labelled "audio". Some AVRs/HTiBs also display the incoming signal via a "display" button (or similar).
As if that's not enough, the speakers need to be balanced for volume output. This is typically done in a number of ways depending on the AVR/HTiB:
- using the internal test tones and a mic
- using the internal test tones and an SPL meter
- using the internal test tones and your ears.
- using a test DVD (like AVIA) and an SPL meter, and/or your ears.
Here's a post useful for those new to this - FAQs, Search Tips, Optimization, etc: