They sound great, just about as good as the Air 2/New iPad 2017... (Not quiet at all.) I suggest trying the iPad Mini 4 in store just to get an idea before purchase. But I recommend the iPad Mini 4 overall, great little device with good power and good quality screen and speakers.
The speakers on the iPad mini 4 are not horrible, but they don't have the sound quality or boast with the iPad Air 2/Pro speakers are. But they are adequate and has decent volume for being such a small tablet.