When you hear a transmission on the channel, press "Hold", then press and hold number 9 until you see "Sound Quality" on your screen. When it starts transmitting again rotate the VFO / Select knob to the left or right to adjust the quality of the sound. When you get it where you want it, press "Resume" twice or "Resume" and "Scan" It does tell you abut this in the manual.
I have the BC785D (base model of the 250D) and I had to adjust my digital sound quality. I have never gotten it to where it sounds like a regular VHF or UHF system, but a least I can understand what they are saying. It sounds a bit muffled and also, the volume of the transmissions is louder than regular systems.
Hopefully Uniden has corrected this problem in the newer BC 796 and 296 models.