Tom Schwartz Singing Karaoke in L.A. Amid Tom Sandoval Affair Drama

Tom Schwartz hasn't shown too much concern about the outrage over Tom Sandoval's affair with Raquel Leviss -- which makes this performance of Green Day all the more worrisome.

Witnesses tell TMZ ... Schwartz was with pals at The Oaks Tavern in Sherman Oaks for some karaoke -- showin' the crowd his version of Green Day's hit, "Brain Stew."

We're told he picked up his group's tab Wednesday night. It's unclear how long Schwartz and the gang stayed out, but we're told Sandoval definitely wasn't in attendance.

It appears Schwartz isn't all that fazed by what's going on with his best bud right now ... but he should be if ya ask former "Vanderpump Rules" cast member Kristen Doute.

As we reported, Kristen -- who dated Sandoval years ago -- completely dragged Schwartz calling him a "f****** p****" for keeping quiet about his friend's affair. She claims he's expressed concern in texts, but she thinks public actions speak louder.

BTW, Schwartz and Raquel were briefly an item too, having made out during the show's current season ... so, there's a little more drama.

Schwartz has been mum on the cheating affair -- up until Saturday, that is ... when he sat on the fence about who to show his allegiance for ... ultimately tossing up his hands and chalking it up to a major bummer.