First, thank you for the many clear english and other languages sandhi examples.  I don't know how you all answer so quickly and with perfect spelling.  Presumably from years of typing up lectures or answering students emails..

Secondly, I want to explain to my  sanskrit chanting but non-sanskrit knowing, non-linguistic audience how and why in the different mantras the word namaḥ is spelled and pronounced differently. Its easy to explain how namaḥ becomes namas in the phrase namas te .
But with the other pronounciations there are some complications to explain.

1) 
namaḥ śivāya is overwhelmingly written namaḥ śivāya and rarely as namaś śivāya or  namaśśivāya . In GRETIL there is  namaḥ śivāya 193 times, namaś śivāya 8 times, and  namaśśivāya 0 times. But I've only heard the written  namaḥ śivāya chanted as  nama śivāya and never as namaha śivāya (i.e. with the visarga pronounced).. I had always assumed that what was happening was that the written  namaḥ śivāya was being chanted as if it was the sandhi transformed namaśśivāya

How do Indians pronounce namaḥ śivāya when they chant it in a hymn or in a mantra?

2) Is there a way to explain the process how namaḥ nārāyaṇāya becomes namo nārāyaṇāya  that would be understandable to someone who wasn't a linguist or a sanskritist. An explanation that would be understandable, but  deeper than just stating the rule: aḥ becomes o before voiced consonants.

 Thanks again,
Harry Spier