I'm trying to get on the same page in an attempt to help. What we are talking about isn't "splitting" but "combining" the mono output from the radio into both stereo channels as they enter your PC's "line-in". Is the "mono" plug at the radio end? If not it should be. The "stereo" end of the audio cable then plugs into the pc.
Not to 'split' hairs here.. but you are technically in error. We want to 'Split" a single channel into 2 channels. You 'combine' multiple elements into one. You 'split' a source into multiple. The device itself may be referred to as a 'combiner' or a 'splitter', depending on which way it is connected. Multiple in and single out, as a combiner, single in to multiple out, as a splitter. Now we'll be on the same page. Also, My Broadwave installs DO NOT 'forget', if the source goes away. Go Figure.??
FLOOD CASTER...You may simply have a defective 'splitter'. Just bad. but, then again, I had one appeared to be defective., but simply the 'molding' of the mono plug didn't allow the plug to fully engage into the Mono jack output of the radio. took care of that with a knife.
2nd make sure its a 'mono' plug.. a 'stereo' plug may not make contact. It's possible to err and get a 'stereo' to 'stereo' adapter, mistaking it for a 'mono' to stereo splitter / combiner. I've done it, lack of attention of old brain cells.
Make sure it's the right size... the earphone jack may be 1/8 inch, while your plug adapter may actually be 3mm, that's just a hair smaller, and may not be making contact... especially if the jack has been used previously with the English size. It's also possible that an original 3mm jack has been 'stretched' by using a 1/8 in plug previously ...
I've been down the road fighting with all these variable mechanical issues.
NOW, something similar may have occurred on the PC end, if you've the plug inputs on the PC.. 3mm trying to fit 1/8 inch stretched! Or attempting to plug a 'mono' into a stereo jack!
Use the LINE IN on pc if possible, rather than MIC...
Mike