The built-in sound systems on many computers are not very good for this purpose. Unless the original recording quality is poor it's worth using a device with some claim to quality for analogue-to-digital conversion; either an internal sound card or an external USB audio interface.

BTW that hum is 50 Hz plus harmonics (and across the Pond it would be 60 Hz), not 50 kHz.

The sound-editing software should offer various filtering options. If it doesn't allow you to apply a set of notch filters at 50 Hz, 100 Hz etc, try another one that does.