Andreas wrote:

I was looking at a chart of reflexes of PIE and Proto-Semitic consonants the
other day, and noted that while, say, the ancestral velar stops are mangled
wantonly in many daughter languages, *m and *n are perserved in every language
listed (in initial position at least).

Now, is this just a quirk of these particular families, or do different sound
have differing "intrinsic" probabilities for changing?>>

Well, let's think about the possible sound change that could happen to initial segments:

1.) The sound could be lost entirely.  I'd say that sonorants are less likely to be lost in initial position, and n and m are sonorants.

2.) They could devoice, if voiced.  Nasals don't have a salient voiceless counterpart, so they have nothing to devoice to.  While it's entirely possible, it's highly unlikely in initial environment, because what would cause such a devoicing?  If a vowel follows it couldn't be assimilation--it'd have to be a very odd case of dissimilation.  The only way I could think of them devoicing is if they were followed by a voiceless fricative--particularly, /h/--and this sound change *is* attested.

3.) They could voice, if unvoiced.  They're not unvoiced.

4.) They could aspirate/deaspirate.  This ties into (2), I think.  While it's not impossible (I don't think), I'd say it's highly unlikely, though could arise with a following /h/.

5.) They could become fricatives.  This happens with mutations in Gaelic (/m/ -> [v]), but generally not independently.  In order for it to happen, a nasal, which is [-continuant], would have to become [+continuant].  This is easy enough for oral stops, because it only involves one feature.  However, nasals are *necessarily* [-continuant], which means that you'd have to drop the feature [+nasal] as well.  In order for this to happen, there'd really have to be some sort of a plausible conditioning environment, and initially is not likely--it's not even that likely for oral stops (though it becomes more likeley if preceded by a V-final word in speech).

6.) They could become oral stops (i.e., /m/ -> [b/p], /n/ -> [d/t]).  This isn't likely, but not impossible (I'll discuss why below).  Why is not likely?  Word-initially is not a conditioning environment that would give rise to a spontaneous dropping of nasality.  I actually can't think of one that is...

7.) They could become some other sonorant (i.e., /m/ -> [w], /n/ -> [l]).  Now I'm not sure about /m/ -> [w], but /n/ -> [l] is *widely* attested.  Take French/English, for example.  Our word "level" comes from Old French *nivel which came from an old n- initial Latin word.  Currently, I believe French has niveau but we  have level.  And, in fact, it's very common for both /n/ -> [l] and /l/ -> [n].  This can't really be explained phonetically, but there  have been acoustic explanations offered--namely, that the difference between [l] and [n] to the ear is negligible, because both are [+coronal], [-continuant], [+sonorant].  The only difference between the two is that one is [+nasal] and the other is [+lateral].  Also, taking this a step further, there are attested sound changes in Bantu whereby /l/ -> [d] or [r] in certain environments, so /n/ could in a circuitous sort of way become [d] or [r], and from there, the sky's the limit.

8.) They could metathesize (e.g. /ma-/ -> [am-]).  I don't have an explanation for why this would be salient or why not.  The cases of metathesis I know of never took place word-initially.  It might be interesting to look this up.

9.) They could neutralize (e.g. /m/ -> [n]).  This, in part, happened in the Polynesian languages with *N (cf. *N -> n in Hawaiian everywhere), but never with [m] and [n].  I'd say there's an acoustic reason for this, and, indeed, an acoustic reason why labial segments, in general, tend not to pattern with coronal segments.  The audible difference between coronal and labial segments is too great for the two ever to be confused.  This is why, for example, the pull chain in Hawaiian didn't affect /p/ (that chain was: (1) Glottals Deletion: *? -> , *h -> ; (2) Fricative Neutralization: *s -> h, *f -> h; (3) Oral Stop Pull Chain: *k -> ?, *t -> k *but not* *p -> t).

10.) They could assimilate to/dissimilate from the following segment.  Again, if followed by a vowel, this is a negligible change, though it should be noted that *n -> J / _[i] is very common.  This becomes more interesting, though, if you look at languages that allow other things to follow onset nasals.  So, for example, what about *mw, *mj, *nw, *nj?  The first two generally produce no affect (the former in Bantu languages, the former in Russian), but the second two certainly do.  *nw -> Nw is quite common (though it could quite possibly become mw), and I think that *nj -> J or ->Jj goes without saying.  Now, depending on how you count initial nasals in Bantu languages, they change all the time--or at least /n/ does: *n -> n (before alveolars) -> J (before palatals) -> m (before labials) -> N (before velars) -> n_0 (before /h/--I hope that notation means "voiceless").  Anyway, for dissimilation, well, the nasal would have to lose voice, if followed by a vowel, or become [-continuant], or something.  Highly strange.

Anyway, there are other possible sound changes, but these are the most pertinent ones I can think of at the time.  A lot of it has to do with /n/ and /m/'s status as nasals.  However, a question to ponder might be why *N so easily becomes [n].