It will likely scare them the first time as it’s new/unfamiliar, but they’ll begin to normalize it with repeated events (as long as there is no physical/situational aggressiveness toward them). Once they acclimate to the singing, they might still get a small jump scare if you begin abruptly belting out something, but as you continue the song they would likely relax back down recognizing it as not a danger. I find most pets will try to hide/runaway when they don’t like a sound, so indifference from a pet is akin to general acceptance of your presence imo.

Here’s wishing you a wonderful performance!