Does a sound card low pass filter input?
It's always wrong to generalise ;-) but I think it's safe to say that all PC sound inputs today and for the past few years use sigma delta converters. These sample at a very high rate, and then decimate to the required lower rates like 48k, 44.1k, in DSP in the sound chip. 

The fact that the actual sampling rate is very high means that a trivial low pass filter at the input will suffice to pass the audio band while rejecting above half the sampling frequency.

When the rate is dropped to the final rate, the use of digital filters means that a very sharp filtering function can be achieved, DC to 20kHz passband for 44.1kHz sample rate is straightforward.