@modern pronunciation of the visarga

Every letter in written Sanskrit has always the same sound, except the /anusvAra letter and the /visarga letter.

So the fact that the /H has several sounds is extremely confusing. Beginners should follow the following guide when reading aloud.

when the /H is at the end of a verse or half verse or sUtra, when chanting

You must add a long echo vowel. Don't ask me why, but everybody does this. As far as I know this is a custom that has been followed since vedic times. /pANini does not mention it.

written: yogaz citta-vRtti-nirodhaH || योगश्चित्तॱवृत्तिॱनिरोधः ॥
chanted: yogaz citta-vRtti-nirodhaHAAAA

when the /H is before a pause but you are not chanting

Pronounce /H as a hiss.

pAdaH पादः → pAda पाद hiss

Some people add an echo vowel after the hiss. This is not PC. Many people do it nevertheless.

pAdaH पादः → pAdaHAAA

when the /H is before k क् kh ख् p प् ph फ्

Pronounce /H as a hiss —

prathamaH pAdaH प्रथमः पादः → prathama hiss pAda hiss

Most of the time you can use kupvoKkaFpauca too —

prathamaFpAdaH प्रथम⩆पादः

Some people add an echo vowel after the hiss. This is not PC. Many people do it nevertheless —

prathamaHAA pAdaHAAA

when the /H is before z श् S ष् s स्

Always apply vAzari —

namaH नमः + zivAya शिवाय → namazzivAya

anywhere else

If /H appears anywhere else, it is a mispelling. Someone forgot to apply the sound change, or to write some punctuation after the /H to show that there is a pause there. When you see this, add a pause after the /H, or apply the right sound change.

guruH brahmA गुरुः ब्रह्मा → misspelling for gururbrahmA गुरुर्ब्रह्मा
or mispelling for guruH | brahmA गुरुः । ब्रह्मा