Does the height of the swirling tea fall inside the cup as the vortex diminishes in size?

I was unable to duplicate the effect.

I assume that tea&tap means tea and tap water. I used a thick ceramic mug.

Though the height of the liquid would theoretically, go down as it cooled and evaporated, no change in pitch was detectable by this retired audio recording engineer.  Pouring off some of the water, of course, did cause the pitch to rise.

The mug could have internal stresses that are unique in this situation, perhaps as temperature equalizes between the inner and outer surfaces of the walls of the mug. The type of mug is not mentioned. If made of glass such internal stresses would be common, and could account for what you hear.

It's a bit much to hope for a video to be uploaded to reveal all the details.

 Since the words level, and pitch, are used interchangably on occasion, and since psychoacoustic effects make louder notes seem sharper in pitch as well, this may be the explanation.

We could beat this to death with scientific dissertation,  but we will be no closer to an answer without more information.

I read the question like this:

• (S)he makes tea
• Stirs the tea
• While it is swirling around (s)he taps the cup with a spoon
• Repeated tapping produces a note with falling pitch

As for pedantic and wordy regulars, can I leap to the defence of all those people who generously share their knowledge here? After all, no one has to read what is written.

In answering questions, we all have our own motives - passing time, meeting the challenge, gaining kudos and worldwide recognition etc. - but it's just a bit of fun. The worst thing is finding your answer corrected, which can slightly deflate your pride, but it does encourage you to answer correctly the next time.

I'm assuming you're stirring after you've added milk. When you stir the hot water and the milk, they form a mixture, essentially by osmosis. To start with the concentration of water is 1. As you add the milk to the mixture the concentration is reduced because the solution is diluted.

Now, the milk you've added is more dense than the water. So the solution is also increasing in density. As the density increases, the vibrations made by tapping your spoon against the rim of the mug are vibrating against a substance increasing in density. So the wavelength of the sound waves is being stretched as they have to travel through the more dense liquid. Therefore the pitch of the note you hear gradually drops.

This can be observed more easily by stirring the tea whilst adding milk simultaneously and making sure you clink the edge of the spoon against the inside of the mug. The drop in note is even more noticeable.