hm, ich have some problems to understand which audio signals of common synths may come close enough to exceed the Nyquist frequency. This must be a signal above 22kHz - me I could not hear that, maybe my little daughter, six weeks of age… 
I assume his problem is not (classic) aliasing, more the jittering due to incorrect adjustments of the recording settings. Maybe some very short kicks of the drumset cause very short clippings of the converter that occur as jittering when playing back the samples. The input LED won’t show a very short clipping.