Ok, let's try some order again.
Please consider that when it comes to such things, TGM doesn't test ANYTHING and leaves ALL the hardest matters to me, so bear with me but it takes time

(this will have to change for version 2

)
I placed some highly visible effects to see how these Songs really work.
The effect is reapplied every now and then, but not continuously.
Now that all the Songs use a projectile (like original Skalds' song), the caster himself is affected as a target of that projectile. If he moves, chances are that he exits the area of effect created by the
last casting, so he'll lose the effects for a while until the song is applied again.
This is my interpretation at least. So the glitch can only be "smoothed" by raising the area of effect...
But I'll leave this to Lingering Songs only, creating a new projectile for those (hoping it'll work..)
About stacking: aVENGER already used a trick for this but if I'm not mistaken it only affects the caster.
With a few simple changes it will affect everyone, with this result: different songs will stack, identical songs won't, no matter who cast them (clones, other bards...)
Actually I could also use other combinations (ex: only one of the songs may work at a given time), but I think the easiest solution is also the best here.
The caster can be made immune to all of the effects, if needed, but I see no real need: if he sings he can't attack, and why removing from him protections and immunities, while other party members do nothing but hearing to get them?
If he stops singing to attack, he'll have the benefits for a while... like the rest of the party, again. Why can't he be exhalted as well?
About animals. Now this is tricky. To exclude them, I'd have to do something like that:
-Use IDS/TARGETING in conjuction with the projectile.
-Use lots of such effects, to target everything *BUT* animals...
-Those spell effects could only call an EFF file, so to place the actual song's effects a new spell should be called from it.
Nothing terrible, in itself, but given the nature of the songs' timings, I don't know what would really happen in the end. Is it really wanted? Can't we consider songs' effect as something magical, in a way? Bards use magic after all
Last problem: all of these modifications (except the one to Lingering Songs) would disrupt coherency with aVENGER's "lesser" songs, since we are modifying only the HLAs.