Your symptom could be contributed by a few things but a bad cable is not one of them (at least not the cause of bad response from clicking the stop button). There is no data-handshaking on digital audio transmission so the PC doesn't care about the cable or the other end. That means you don't get bad response from clicking the stop button just by a bad digital audio connection.
That leads to my suspicion on your playback material. What is it? Since you expect 5.1 output, I suppose you're playing back 5.1 material. In that case, make sure your player sends audio to SPDIF (or passthru' as it's called sometimes) if the player uses internal decoder. If the player uses external audio decoder, make sure the external audio decoder sends audio to SPDIF.
If you're talking non-5.1 signal, then only soundcard with DTS Connect or Dolby Digital Live can convert non-5.1 signal to 5.1, or you need an audio decoder to convert non-5.1 signal to 5.1 and then "passthru'" to SPDIF (e.g. AC3Filter, ffdshow audio decoder).
Or if you're talking 7.1 signal, your player needs to know how to downmix signal to 5.1 stream. From my experience, Cyberlink PowerDVD (Bluray/HDDVD support) has been finicky on that on earlier versions.
There're other possibilities like video file has bad audio/video syncing but in that case I assume you've tried a trusted source like a commercial DVD movie.