On a typical older 5.1 computer sound chip,the main stereo output channels have 32 ohm drive, intended to run headphones from the chip when it is set to two channel (stereo) mode. The other four channels, (surrounds, center, and sub), tend to be 600 ohm output impedance, which is a lot weaker.
Nowadays i have heard they have impedance sensing devices...so it just works fine.