I don't know if Elon Musk or SpaceX has ever kidnapped a seal and played headphones to it, but the photos that were going around the internet for the last few years (they made the rounds in 2017 in connection to SpaceX, before Musk began referencing it in 2023/2024) were from an unrelated study in 2006: https://x.com/mcrs987/status/1848070131781455911
I'm still looking or the original study to find out what the "larger study" was about [was it an impact study related to Vandenberg? Was it part of seal monitoring, and while they had the seals they did a bunch of other stuff?], so if someone else digs it up I'd be interested in a link.
I don't know exactly how often who puts headphones on seals yet, but it sounds like as part of operating at Vandenberg, it's routine to "haul out" (by which I assume they mean, capture and remove, releasing some time/distance away) harbor seals, so they aren't in the way; it sounds like they also (sometimes? always?) perform some amount of monitoring activities on the seals, since you've already got them captured.
Also, all of this activity is apparently termed "Level B harassment".
I'm still looking or the original study to find out what the "larger study" was about [was it an impact study related to Vandenberg? Was it part of seal monitoring, and while they had the seals they did a bunch of other stuff?], so if someone else digs it up I'd be interested in a link.