Oh yeah, my 7.5 month old has really started doing this too. I can tell he's thinking, "La la la, I can't hear you! I don't want to so I can't hear you!" So frustrating!
I just make sure not to ask him to do anything unless I'm ready to enforce it. If I ask him to come and he doesn't, I get my butt off the couch, go get him, and bring him to where I was. (If he runs, then I run the other way to get him to chase me.) If he comes, no matter if he takes his sweet time, he gets praise. If he comes quickly, lots of praise and maybe a treat. Same with sits, downs, relax, don't chase the cat, etc. It's like it's back to puppy training!
And I only ask once - this is important because if you ask multiple times, you're just asking him to ignore you.
Like Hineni said, stay consistent, be firm when necessary, and be patient! (Oh, it's so frustrating!!)