This stimulates both the ventral and dorsal branches of the vagal nerve. If we have been more shut down, this can indeed release that emergency brake of the strong dorsal response (freeze), by stimulating the healthy low-tone dorsal, or social engagement systems of the ventral, both of which help to down-regulate the sympathetic in a gentle way. If that emergency brake has been on, then this tapping into a more gentle dorsal response could actually elevate the heart rate.

Another explanation could be that this sound actually tapped into your sympathetic nervous system, but given the relatively small increase, and that at 80 bpm you’re still in the normal range of healthy heart rate while at rest, I’m going to guess it was likely the first explanation.