My attempt to translate these descriptions into not quite linguistic terms.
 
I believe that for 'Nearly Silent' the authors are referring to the fact that the Final -R inflection is a non stressed syllable. To a non linguist this means that it makes less noise than a stressed or medial syllable. Similar to butter and rubber. Though it is also shorter. Not trilled ( as in classical languages)  or held (like in bar room) but short like in 'short' if you didn't say the -t-. and not like in shore.
 
And, if i recall correctly, the inflected -R ending on runestones in represented by the 'pitchforky one' "erhaz" in one text. This used to be a -Z- sound but over centuries is became zh..then rzh, then R, then faded entirely.
This is why the rune names are often ending in Z such as tiwaz, erhaz, thurisaz. This is the Older pronounciation.
There is more of this on  Vikinganswerlady.com.
 
Ambrosius- Who has yet to find an appropriate Norse name, though Gummi Bera is sounding appealing.