Hi Chemix,
We have a short list of Audio Monitoring Options with DeckLink. The Flying Cow convertor is another option which you could use with your existing equipment.
I would advise against a sound card as that would add latency so that your video and audio would be out of sync.
Because you have fast dual-processor G5 with PCI-X, the DeckLink SP will be able to downconvert your HDV material to SD for output to your Betacam SP and DVCAM decks. Just use a Blackmagic HDTV 1080/50i HDV easy setup in Final Cut Pro HD 5.0.2 and play the video. The DeckLink SP is a standard definition card so it will automatically downconvert and output the video as uncompressed PAL video. You can use the DeckLink PrefPane in the System Preferences to choose between letterbox and anamorphic down conversion.
Alternatively if you are wanting to apply RT effects, or mix your HDV material with DV material on the same timeline, you would do well to use the down conversion in your HDV camera to output PAL DV. Capture this directly on to a PAL DV timeline in FCP and then you can mix it with any other PAL DV clips. Edit and apply any effects that you want and then output to tape via the component analog outputs of the DeckLink SP using the Blackmagic PAL DV easy setup.
Regards,
Luke Maslen
Blackmagic Design