I had no idea how to pronounce it because none of the languages I know have an h sound that is unvoiced. In fact, I don’t think there are any living languages that have an unvoiced h.
What about English?
In Sanskrit, ह is voiced, which is actually different to the English
h, which is not voiced.
The traditional pronunciation of
visarga is /h/ (as in English unvoiced /h/), followed by a short "echo-vowel". So इः is pronounced
ihi, with the second
i being very short, as in ॐ शान्तिः शान्तिः शान्तिः you may perhaps have heard the famous chant pronounced
Oṃ śāntiś śāntiś śāntihi.
The original pronunciation was likely /h/ (as in English unvoiced /h/) with no "echo-vowel" at all.
All that I have just said refers to
visarga at the end of a sentence or
pāda.
You may know, if you've studied Sanskrit a bit, that sounds change a lot depending on what follows. So in Classical and Vedic Sanskrit it is pronounced
s before स,
ś before श (see above) and
ṣ before ष.
In Vedic it was likely pronounced similar to Urdu خ before क and ख and like an f, but without the teeth actually touching the lips (IPA: ɸ), before प and फ. The first pronunciation is also known as
jihvāmūlīya and may be written with a different character and the second is called
upadhmānīya
