1) The sound, during sex of any kind that involves a phallus (organic or otherwise), made by its quick (& sometimes painful) ejection from the cavity in which it was plunging. The resultant 'POP!' when echoed inside the cavity, is the baritone seal. The POP is, much like a freshly-opened pickle jar, the breaking of the seal.
2) The seal-like moaning sounds made, usually accompanied by sealion-esque bucking and thrashing, by certain types of sexual partners nearing orgasm.
1) "I was getting deepthroated by Theresa last night when I got excited and slipped out--you should have heard the baritone seal it made, bro! Sounded like a hotdog bustin in a microwave!"
2) Yo, I tried to hook up wit Robin last night, but she started letting out baritone seals when I went down on her--bitch was putting on a Sea World show, dawg!