Ah, right you are. 
Of course, as I’d mentioned earlier that while it’s definitely a feature, the practise for me is by no means across the board, but only with certain words. I don’t even necessarily know what they are until I utter them! 
this is terribly interesting!
if I had a sound clip I might be able to work out what yer on about…
C’mon, Denny…it’s been explained pretty clearly, I should think. If someone can help Denny out with a sound sample, by all means go ahead. I would, but I don’t have the tech, myself.
while I understand that you’ve gotten some people to know what you are talking about 
I’m just sayin’ that it is not working for me.
Eh, sorry. Dunno how better else to put it across, yet.
Okay for “Moth”, there are people who will pronounce the plural and possessive with a voiced “th”.
Actually it’s much the same for “Math” and “Maths”. More people would used a voiced “th” for the plural, at least in the UK.
That makes me think that that is the template for people’s pronunciation of “Moths”.
So what about “Myth”?
Always hard, I’d say, and the same with “Meths”. (Is there ever a “Meth”?)
… but only in the UK would anyone actually say “maths”. On this side of the Atlantic, both north and south of the border, “math” is a collective noun.
Never heard anyone do that!
Nor me. Not ever.
Just read what he said about the frikkin’ fricatives.
Tsk. Sometimes I think I’m the only one who listens… 
For myself, the same as Peter and Ben. And for Maths.
I vaguely remember hearing it the other way for the plural a long time ago. It was a time when I was fresh away from home for the first time and hearing many strange and unfamiliar things. I had yet to meet any Americans.
I just now decided to do a rough inventory of nouns ending in unvoiced TH that I habitually voice in the plural, and out of all that that I found (eliminating compound words), the basic list is comparatively small:
Bath, booth, cloth, moth, mouth, oath, path, sheath, swath, truth (and now, Goth! 
). I don’t think there’s much else.
Weird.
I just can’t even force myself to say the following words with a voiced ‘th’: cloths, moths, oaths, sheaths (it’s a different word from sheathes, with different nuances), truths and Goths.
I struggle to use a voiced ‘th’ for baths, unless that word is a plural noun, in which case I would always use the voiced form. I have a mild preference for the unvoiced ‘th’ for booths, although I would always use the voiced ‘th’ for mouths, whether that word was used as a verb or a noun. I prefer the unvoiced ‘th’ for paths, though I probably sometimes voice the thing. The plural of swath is a cheat, isn’t it? Because surely it’s spelt swathes rather than swaths, which makes it different, and of course leads to a voiced ‘th’.
Baths, cloths, paths - unvoiced, no doubt.
Booths, mouths - voiced, no doubt.
oaths, sheaths - probably voiced but wouldn’t hear it as odd if unvoiced.
truths - not sure, awkward. If making presentation, say, probably unvoiced and the 'r rolled more than in the singular.
swathes - voiced, without looking it up I would spell it that way for a different ‘a’.
(crossing)
Edit to add, after reading Ben’s post, that I probably come from the other side of the isoglot where the ‘a’ in bath, glass etc changes and this may be relevant to baths.
(just off to see if there is a Beeb iplayer link the the progamme they played on Sunday where I learned the word isoglot.)
yes, here we go: http://www.bbc.co.uk/iplayer/episode/b01ljwm4/The_arse_that_Jack_Built/ (spot the presenters slip of the tongue near the end)
Apparently the singular word is either “swath” OR “swathe”, in which latter case it is of course pronounced not “swoth” (unvoiced) but “swaythe” (voiced). I’ve never used the latter. The dictionary I just sourced indicates in neither case a spelling change upon pluralisation; just add an S, so according to that entry the plural of “swath” ought not to be “swathes”. Interestingly, in the same source the plural “swaths” is attested to be pronounced [swɔːðz], just as I do.
Must be a Yank dictionary.
What slip of the tongue, David? I thought I were listening careful like …
I tried to catch it too. I couldn’t be housed to listen again, though.
I’ll listen again just to check, but I think that when they were in the street near the end the presenter - from the house/out side of the line - said that they were " looking for someone to help us art". Witty, but I thought it went on a bit too long.
To add to the above:
swaths - unvoiced.
booth - singular voiced, so plural voiced is consistent.
yes, here we go: > http://www.bbc.co.uk/iplayer/episode/b0 > … ack_Built/ (spot the presenters slip of the tongue near the end)
That kind of explains the TV show “House”.
\
Edited to fix Quote function mix-up. No idea how the names got switched.