Elaborating a bit. I would use three resistors: one to act as dummy load for the amplifier, and then two to make the attenuator:
Word of caution. If the speaker is driven from a bridged amplifier, you cannot hook SpkrL to the grounded input to the sound card. Unfortunately, computer sound card line and mic inputs use the computer case as a common ground, which is connected to earth ground via the power line. I have had to use a transformer to provide isolation to solve this problem.