I've been using Ableton with ASIO and Presonus AudioBox USB 96 since many years and I've never had this problem:
The sound line in sounds distorted (my guitar, the mic, etc.). I bought a Casio PX 310 and plugged it in the MIDI entrance of Audiobox soundcard, and since that -I think- I have this problem. When I turn the Audiobox soundcard mixer wheel to "playback" the guitar or the mic (everything that it's plugged in the line in entrance - both of them- sounds very distorted), but when I turn it to inputs it sounds clear.
I've changed the buffer size many times, tried it with ASIO software and with Presonus, and tried many configurations in Ableton (also WIFI and Bluetooth disabled). I also tried to plug in the Presonus soundcard in different USB ports and ASIO and Presonus software is updated.
It always worked, but since I plugged the MIDI it stop working, is it possible I change the MIDI config and it make the playback signal distorted?
Everything on my computer sounds good (music, youtube videos, etc.), and Ableton projects, MIDI tracks (also played from the casio) and tracks sounds good too, it's just the audio line in playback.
Thanks in advice (and sorry for my english, it's not my native language).