Afaik is not normal, that the amplitude will change depending on the change in pitch. When the step is set above the original, openal speeds up sampling by a factor (afaik), using some interpolation when the multiplier is not integer.
There may be some rare cases where the amplitude changes, but probably not for longer samples with a lot of frequency content (as is usually the case with most natural sounds)
How loud we perceive that the amplitude depends on the pitch, see equal volume contour
Maybe this effect explains your question?
As a workaround, you can lower the gain for regular sounds and use a higher gain for higher sounds.
Or multiply the source data by a factor before attaching / passing to the buffer.