Well, a few questions.
1: What radio do you have (See note below)
2: How are you feeding the computer's audio to it
3: Go to Device Manager on your computer, find the USB - RS-232 converter (it is under PORTS) and tell me what chip set or what other info is there.
Notes: Some radios like the Kenwood TS-2000 which PTT line you key (Front or back) decides what audio input you use, The CAT (Serial cable) Keys the FRONT line only so you need a 2nd adapter keying the rear PTT using the CDS/RTS line(s) to make it work
2: See 1
3: As I said up-thread, I have 3 adapter shere, the Prolific won't even download frequency to YP-LOG The belkin will do that but won't let me read the radio's memories (or write them) using the TS-2000 memory tool from Kenwood.
The RTS-3 from RT-Industries works, period, end of line, everything is proper. NO complaints.. There are several other RS-232 <->USB adapters that work as well but I don't know which ones.. Several threads here list them.