Check your sound chain to see if at any point a compressor is inserted. It is not uncommon for PC sound cards to have compression enabled for the inputs by default.
In this scenario at some point the input volume would be too high, triggering the compressor to reduce the aamplitude of the input signal to avoid distortion. After triggering this would then take some time to fade back to the uncompressed volume, where in all likelihood it would be retriggered. This would most likely be characterised by a sudden drop in volume and a slow return to original volume.
Solution, disable the compression, or adjust the threshold and type of compression
or
turn down the input signal so that it isn't triggering the compressor/limiter