Time spent lying in semi-supine is time dedicated to releasing unprofitable holding patterns, and promoting freer, easier muscles, joints, and breathing. It is time spent practicing in an undemanding situation, the skills of poise and level-headedness valuable when the pressure is on.
Practise is good any time. Special times might be when experiencing neck, back or other pain; or if you are tired, this is a constructive rest position (much better than slumping on the sofa!).
You cannot learn the Alexander Technique from sound files! Good teaching plus time are needed to bring not only the benefits in functioning that are possible, but also a sufficient grasp of how these improvements have been made, in order
to be able to continue to maintain and develop benefits independently of a teacher.