Hello,
You actually have part of the answer in your question. It is not obvious in the UI, but the IDs in that Voice Inspector columns are a mixed bag: When beside a Game Object (with the Game Object icon) the ID is the Game Object ID, which can be used in the Game Object Watch (hence the name). The other ID you see (the "short one") is the instance of the Sound (if you play several times the same sound you will get a different number). It can't be used to watch anything and only serves to differentiate sounds within the Voice Inspector, when you have multiple copies playing.
RTPCs are applied on Game Objects, not Sounds. If the Game Object Watch report a value for the RTPC, it is the one used by all Sounds played on that Game Object. You can compare that with the RTPC value reported in the right-most pane of the Voice Inspector, when you have a particular sound selected, and it should be the same.