Hello,
I have a USB audio interface (3D Sound Audio Interface see below) plugged into a raspberry pi. When I have it selected for audio input (port audio seems to be the only one that works) along with audio output via the speaker on the GPIO pins I get crackling as if PD can't handle the processing of audio input while outputting on raspberry pi. NOTE: if I turn off audio input the output sounds perfect.

Anyone else encounter this issue?

FYI, I hooked up a nice little speaker to make the Raspberry Pi more portable via the I2S Amp on connected and powered by the GPIO pins (see this link for details

@mpromeo I don't have much confidence that I will help you but......
Try increasing the buffer for audio by setting Delay(msec) to 80 or more.
If that solves the problem then you can decrease the setting until the problem reappears and then set the minimum where it works.
That will increase latency though.
Power could be a problem as the mic input is in a usb device.... You could try attaching the usb soundcard through a powered hub. Is it 44.1K capable..... does changing to 48K help?
((that will not be a solution if all your recordings are 44.1K as Pd will play them at the wrong speed))
And any other usb devices plugged to the rpi at the same time, especially data drives, could cause problems for usb audio I/O.
David.