I'll admit, I thought this was a bonkers theory at first. It's still bonkers. But after listening to the Avengers: Endgame clip in question multiple times with the volume up, I too can hear a whispered "I have telepathy" as Thor (Chris Hemsworth) approaches Captain Marvel (Brie Larson). It's right after he puts down his bread to approach her and watch her hold strong as he summons Mjolnir.
Conspiracy theories aside, it sounds like that was not intentional from Avengers: Endgame directors Joe and Anthony Russo. In fact, Joe Russo laughed when he was told some fans were convinced Captain Marvel tells Thor "I have telepathy" in that scene:
[Laughs.] No. There's no truth to that whatsoever. We buried a lot of Easter eggs in these films over the years, but that was not one that we buried.
So "I have telepathy" wasn't one of the many intentional Easter eggs in Avengers: Endgame -- for Thor or less prominent Marvel characters -- per co-director Joe Russo to THR. But that doesn't mean it doesn't exist 'cause I just heard it too. Granted, I'm an X-Files fan who wants to believe, and it's possible I'll have an SNL-worthy abduction story to share later.
Some fans can't hear it at all. Some fans think people are confusing Thor's breath with the "telepathy" line. But no. Thor breathes, but you can also hear a woman's voice saying "I have telepathy" if you put the volume up and listen closely. It sounds like it's just a really bizarre coincidence, but it was something that started working the rounds in the rampant speculation right before Endgame's release.
This is not the first time fans have speculated about Thor and Captain Marvel. Fans have debated which of them is more powerful (Kevin Feige named another Avenger as more powerful than both). And some fans wondered if Thor and Captain Marvel might launch a relationship after that "I like this one" scene. That doesn't seem to be the plan at this point, although Thor: Love and Thunder director Taika Waititi doesn't seem pumped about the Captain Marvel/Valkyrie fan theory either.
Keep up with all things MCU -- whispered via conspiracy theories or not -- through our handy MCU guide.