Mandukya Upanishad is the smallest and most complex of all the Upanishads. There are just 12 Shlokas which attempt to explain the infinite One, the abstract One!
Mandukya discusses A-U-M(not the sound) to understand the anatomy and physiology of mind.
Want to get a glimpse of that Abstract? Anyone can feel That using AUM as a sound. :)
Here goes a simple exercise to feel That:
Pre-requisite: You should not be a deaf (I guess).
1. Get away from distractions (mainly audio ones). If you have that will and control then you can choose to be on MG Road or Airport road also.
2. Become aware of your surroundings: the babes and hunks (read boobs and dicks), the eternal motor noise, the temperature of your cheeks...etc etc. This is a simple exercise to calm you down.
3. Now once you have calmed down, realise that you have calmed down. Go to step 2 if you haven't really calmed down (Developers, please excuse me for the evil usage of GOTO).
4. Closing of your eyes might be helpful.
5. Now inhale and say A(A as in Upper) and "feel" that A is vibraing and sourcing from your navel.
6. Without any discontinuity convert that A sound into U(U as in Oomph) and raise your focus to your heart.
7. Without any discontinuity convert that U sound into M(M as in the bumble bee's humm) and raise your focus to your highest point(top of your head).
NOTE: All these sounds are being generated within a lungful of inhaled breath. Naturally while you sound you will exhale. Just go slow. Be there for that extra second at each stage of moving upward.
8. Be at M till your breath thins out. Let it thin away gradually (no digital leap please). Hold your focus at the top point of your head. Just be there and as soon as your M fades you will feel a Silence, a Silence different from the normal regular silence. It wont last long but you will clearly feel That in that Abstract Silence.
Thats just a glimpse. Imagine dwelling in that Silence for long (well... time doesnt exist in that Silence but still my language is just trying to express the unexpressable)!