If you watch Pawn Stars and have all of a sudden noticed that Chumlee's voice starts sounding different all of a sudden half-way through the series, then there's actually a kind of unusual explanation:
Apparently it's all down to the fact he's lost weight and also had his tonsils removed.
If you're looking for some proof, all you need to do is scroll down Chumlee (aka Austin Russell's) Twitter timeline @chumlee_
On the 2nd of November 2013 he tweeted this about his change in voice:
SO there you have it! The mystery of Chumlee's voice change is really down to dieting and tonsillitis, and now he sounds like a young girl.