Well there is no doubt that both have a considerable overlap, both in meaning, and in usage. However taking them literally they are not really synonyms:
is a general purpose term which may include all, speakers, headphones, Bluetooth, sound system, pulseaudio, jacks, ... (long list).
should be restricted to the hardware, the chips that produce sound (and eventually their drivers or settings).
So "soundcard" is a subset of "sound" (similar to "monitor" - "screen" - "graphics" - "ati" - "nvidia"), really. Having a separate tag "soundcard" may be of some value for people seeking advice for a driver to their card, or for making settings on a specific card. These questions could easily be covered by the tag "sound" too, but when searching it may help to narrow the search for questions tagged with "soundcard".
Not that I can offer a specific question where this matters here, and the differences may be minimal, but I vote to let them coexist. If needed we may consider pointing to the appropriate sister-tag in the tag wiki to avoid wrong use of a tag.
Nope. Out ~140 questions, 107 also have the sound. Is safe to say that they are one and the same (if you are asking about sound cards you are asking about sound, no?) through, soundcard can refer about hardware which also is the main use of the sound tag anyways.
sound tag seems also pretty vague, if Takkat can shine his (her?) wisdom and tell what the tag needs (if it needs something, maybe better tag wiki.... IDK) so it gets better used... we would be glad, right guys?
The soundcard tag is useful for people who have a special interest in that area, such as people who are running Ubuntu Studio. If having a soundcard tag could contribute in any way towards helping people to use Ubuntu to produce creative content, then I am all for it.