Tuning
Why the tuning matters
Most tonal instruments in the West are tuned to the equal tempered scale. A C major scale on a piano is equal tempered. It is a compromise. Each note is nudged off its natural harmonic ratio so that every key and mode works on the same instrument.
Before that compromise a tuning had to choose. Some intervals came out beautifully and others came out badly, and the bad ones were called wolf notes, because they howled. Eastern traditions largely kept those older tunings, and so did the West until about a hundred years ago.
Sound meditation wants natural harmonic intervals rather than equal tempered ones. Plenty of what Hamid plays is equal tempered anyway, including Native American style flute, harmonium and crystal bowls. There are ways of playing them that bring the natural overtones out, and a good facilitator ends up doing it whether or not they know the theory, because it sounds more meditative.
The voice is the interesting case. People do not naturally sing in equal temperament. Hold a sustained note over a shruti box or a harmonium or a bowl and the harmonics build on their own.