Dude, why is Grunty's Lair like, the greatest hub in a video game? Like, it doesn't make any sense. It's incredibly claustrophobic, it pretty much plays the exact same theme over and over (albeit in different variations for different areas), it should suck. But it doesn't. It's quite simply so incredibly satisfying to explore, the music is stupidly catchy (seriously, it was stuck in my head for a week straight after beating Banjo-Kazooie), and I legitimately want to go back to the game and 100% it just to explore Grunty's Lair more.
Banjo-Kazooie is an IP created by Rare, and is currently owned by Microsoft. The song is Gruntilda's Lair, composed by Grant Kirkhope. I did try to edit the song to get it to loop seamlessly, but the project bugged out, so you'll have to deal with that, I'm afraid.