I am having this exact same issue, except my sound quality issues go beyond buzzing and include loud pops and crackling as well.
I am streaming through my DDJ-SR, using Serato DJ, into MixLR. I have my Mac's default output set to "Headphones/Line-out" and the input set to "Line-in". I can play a music file through the devoted MixLR "playlist" channel and it comes through the other end of the stream crystal clear, however if I send the exact same file through Serato/DDJ-SR and into the MixLR "device input" channel (set to my DDJ-SR) the sound quality degrades immensely. It's difficult to trace the source of the issue because the DDJ-SR output sound, when plugged directly into any speaker system, or recorded, sounds perfect. It seems that output from the DDJ-SR is getting distorted somehow after it leaves the hardware but before it enters the MixLR application. I've tried just about every possible combination of input/output settings between my laptop, and MixLR (Serato does not have any options here...) to no avail. It is very irritating - It's gotten to the point that I must record my live sets in Serato and then play the recorded file over my MixLR station so that my viewers can enjoy what they hear without major sound quality issues.