Yes, that will work. Do we need anything else for this ability?
There's just the question of how it interacts with bardic countersong.
I'm thinking a countersong would work as a substitute for the final round's save-vs-death and
maybe allows a save if the target is at 0 or negative hit points (although I'm not quite decided on the latter).
Putting it all together, so far we've got:
Sound of Death: A caprine with a resonance score of 101 or more can play a terrible cacophany on a musical instrument that lasts 10 rounds. This heart-wrenching sound starts harmful vibrations in any living creature who hears the tune and is within 40 feet of the caprine when it starts to play. Starting sound of death is a standard action that requires concentration. The caprine must sing or play a musical instrument each round they maintain the sound, this is a free action that doesn't require concentration.
The caprine can tune sound of death so it affects or spares particular creatures, specifying what individuals, races or species the sound harms or does not harm. At the end of each round, the caprine takes 1d8 hit points of damage, and all creatures affected by the sound's initial vibrations that are within earshot of the tune (a 300 ft. spread) take 2d8 damage. However, this damage cannot reduce the affected creature below 0 hit points. At the end of the final tenth round, all creatures affected by sound of death (apart from the caprine) will automatically die (no save) if they are at 0 or negative hit points, or die if they fail a Fortitude saving throw (DC 10 plus half the caprine's HD plus its Charisma modifier) if they have positive hit points.
If the caprine is forced to stop playing the sound of death before the tune finished on the 10th round, the caprine takes 1d3 Constitution drain damage with no saving throw.
[Bardic countersong?]
Sound of Death is a sonic death effect.
Looks OK apart from the bit in red. We can trim down the wording a bit. I see several sentences that are a bit padded out with extraneous words.