He still kinda sounds like his young self there.
How weird to here a clip of Carolyn Williams...
This is indeed rather strong evidence that the slurring started later on. However, I think this tape is running way too fast. So while Brian does sound very energetic and is not slurring, I think his voice probably didn't sound quite that high-pitched in 1981. He sounds like his 18 year old voice there, and other examples I've heard of his voice circa 1981 have his voice a bit deeper than that.