His unboxing channel is not just audio. He unboxes whatever arrives. People who only care about audio have Z Reviews and don’t have to sit through a AC cooler, water jug, or soy sauce unboxing.
He also does live streams and has a cooking channel.
Having discrete channels with specific topics is also a good strategy for monetization. YouTubers can keep the higher rates for tech channels than cooking or random stuff due to being able to focus advertising.
Also having more than one channel is a way to mitigate the risk of take downs, strikes, and other adverse actions from stopping all cash flow.
He also makes money off of the channel by accepting companies to send him stuff that is unrelated to audio to unbox it and give impressions and gets paid for it sometimes.