the microphone

it is difficult to predict how it will affect the results - needs to be pre-calibrated

quality microphones are expensive

background noise

the microphone picks up background noise and sound reflected from objects in the room

this allows the sound system to be tuned to a particular listening environment

the sound card

some sound cards do not support simultaneous playing and recording

the quality of the sound card may become a factor if its electronics introduce non-linearities during play and record operations