If the infrasound was not removed from the vocal-only track and compression was applied, that could account for the vocal volume abnormally fluctuating : the vocal waveform is superimposed on the inaudible low-frequency infrasound waveform. If dynamic-range-compression is applied to a track with high-amplitude infrasound there will be noticeably more attenuation of the vocal where the infrasound has a peak or a trough.
The first thing you should do to any of your recordings before processing them, (e.g. applying compression), is remove the infrasound with the equalization shown below …
No-infrasound-in-Audacity.xml (187 Bytes)