There are several possible causes of bad sound.There might be an impedance mis-match. The peak to peak volume levels from the PC sound card might be too high or low.
There can sometimes be RFI from the PC leaking through the sound card and into the transmitter. The lack of audio input buffer amplifiers on the audio input side of most low cost transmitters can be a problem, too.
Read more about fixing bad audio here
Sound is all we have to connect with our audience so a little effort here has a big outcome!