There is no change. Those both sound just like if the two words were said in isolation.
I don’t know why you think there might be a change, nor why you consider that a “complicated combination of sounds”. Native speakers would not.
On the other hand, one common pronunciation of months suppresses the th, so maybe that is what you have been hearing. Children are known to sometimes say moss for moths, but they eventually correct this lest they be perceived as having a speech defect.
A following sibilant does not neutralize an earlier fricative, at least in careful speech. In fact, phonemic voicing is maintained in noun–verb or in possessive–plural pairs like:
- cloths, clothes
- breaths, breathes
- mouths, mouthes
- wolf’s, wolves
- calf’s, calves
- house’s, houses
- moth’s, moths
In the first word of each pair above, that word ends in two unvoiced sounds, whereas in the second word of each pair, it ends in two voiced sounds. Sometimes this is (somewhat) reflected in the spelling, but often it is not.
As I mentioned in comments, you should practice correctly saying fifths, sixths, twelfths until you have it down pat. Then once that’s done, you can then proceed to
The sixth sick sheik’s sixth sheep’s sick.
Which is surely one of the most pleasurable phrases in the English language. :)
Ok, I’m just kidding. These can be hard for anybody. But in careful speech, they can certainly be done. Very, very careful speech, perhaps, but not impossible.