As to the change in the pitch of your spoon hitting your cup mentioned in When I stir my coffee, why does the sound gradually change pitch? , the ball was seriously dropped. No attempt to recreate the effect, no experiment, just a hip-shot response.
I have therefore filled in this deficiency by conducting the following experiment:
I got out three coffee mugs of differing types, and one by one added hot WATER (no powders), stirred them, and listened to the pitch of the spoon/mug interactions. They did indeed rise in pitch in each case. I then let them sit a few minutes and tried again: no change.
I did note a reversion to the original pitch upon putting COLD water in the mugs.The pitch of the mug without anything in it, of course, differs from that with anything in the mug.
Conclusion: The change in pitch is caused by the change in temperature of the mug, Jearl Walker’s half-hearted guess be damned. (I would welcome a defense of his response, given second hand as it was.)
Anyone attempting to replicate this experiment would find best results using a mug with heavy walls, like the old Buffalo-type diner coffee mugs, presumably because the added mass and thickness takes longer to heat, slowing the effect.