Guys try to set all at same sample rate. Best is 48kHz. Player, Vcable (whatever it is) ST (even for FM), soundcard!.. etc
You are limited because Leif's RTA and MPX Tool are best with 48kHz, so all must be at exactly at same sample rate.
If you use winamp look my previous post how to set it always at 48. If any other player, you must find way to make it to always work at 48kHz.
Or use XP! xD
you mentioned something like "Input module decoder tuned to 48kHz". To the best of my knowledge module decoder has nothing to do with mpeg. It's a decoder for *.mod file format.
EDIT: Indeed module decoder has nothing to do with that
If you're messing around with samplerate in VB - REMEMBER TO REBOOT
Also I confirm that Bojcha's way works perfectly, but with VAC and not VB. Therefore If you want to avoid problem with samplerate conversion, the VAC is better choice.
So: set everything to 48 kHz following Bojcha's instructions (earlier in this thread, if using Winamp, set out to directsound). Remember to set 48 kHz literally everywhere (playback and record in soundcard settings - don't forget that).
When you want to check FM settings and pass signal to MPX tool you need another VAC, which you set to 192 kHz 16 bit (and set cable format).