@jackiejr you're likely hearing differences in quality of the sound card in the newer Dell and Macbook air vs your older Mac Mini.
The USB ports on the newer Dell and the Macbook Air are probably 3.0 vs. USB 2.0 on your Mac mini but the differences in sound quality can most likely be attributed to the sound card.
The USB ports on the newer Dell and the Macbook Air are probably 3.0 vs. USB 2.0 on your Mac mini but the differences in sound quality can most likely be attributed to the sound card.