It is affected by CHR, skill +, and macc.
In addition to this, with equal wind and string skill, wind instruments are more accurate for debuffing. This is an important point that is not to be forgotten.
Also, song+ on debuffs adds to accuracy as well. Hence, for example, Elegy is more accurate with a Horn +1 (Elegy +2) than with a Horn (Elegy +1) than with a Royal Spearman's Horn (CHR +3).
Song+ also increases potency for buffs and debuffs with variable potency, i.e. not Lullaby, Finale, or Virelai (exact potency change varies from song to song and instrument to instrument). It also increases duration by 10% per song+, for buffs and debuffs with duration (everything but Finale).
Of course in the case of Lullaby and Threnody, these two points come into conflict:
Mary's Horn: level 14, Lullaby +1 (30 -> 33 sec duration), wind
Nursemaid's Harp: level 60, Lullaby +2 (30 -> 36 sec duration), string
(Horde Lullaby also hits a broader aoe with string than with wind. Situationally this is either a good thing or a bad thing.)
Piccolo +1: level 9, Threnody +2 (-50 -> -60 resistance, 60 -> 72 sec duration), wind
Sorrowful Harp: level 70, Threnody +3 (-50 -> -65 resistance, 60 -> 78 sec duration), string
As a general rule of thumb, you're better off using the weaker song+ wind instruments for these songs, since it's almost always better to land it reliably than to sacrifice a fair bit of reliability for a slight increase in potential. There are, however, a few situational exceptions. Sufficiently wimpy mobs (esp. if you've got good debuffing gear) aren't that much less reliably sleepable with string than with wind.
So in a situation like Einherjar, for example, where you want to sleep a lot of mobs but they're not individually all that hard to sleep, you'd be best off using Nursemaid's Harp. In many (most, to be honest) other situations, where it's either small groups of mobs or only one mob, you'd want to use Mary's Horn.
Sorrowful Harp doesn't have the aoe justification, so it basically boils down to having a harder time landing Threnody in order to get an extra -5 resistance out of it. Since you generally only *need* Threnody for things that are resistant in the first place (HNMs for example), they're gonna be resisting the Threnody itself too, rendering Sorrowful Harp's usefulness very minimal. You pretty much always want to use Piccolo +1.
Also, the song+ instruments for Finale and Virelai are both strings. They're also both silly investments. The only thing in favor of the Military Harp (Finale +2) is that it reduces the resists on Finale due to the song+... except its being a string will counteract that. -1 inventory and -howevermuch gil... right. Use any old random wind instead. (It always surprises me that people still buy this POS, let alone pay ~150k in today's market. You'd think bards would've gotten themselves educated by now.)
As for Virelai and Cythara Anglica +1 (Virelai +2), the same argument applies, except you get 6 more seconds of duration (so now instead of an uncontrollable pet for 30 seconds with a 60 second recast, you have an uncontrollable pet for 36 seconds with a 60 second recast, woohoo!). You could maybe make a case for the improvement, esp. since you probably won't be trying to charm anything high enough level to be resistant... But Virelai just plain sucks (except as super-emergency crowd control and a way to reset a charmable mob's sleep resistance, hi2u Ziryu, neither of which you really need the instrument's duration for), so why bother carrying an instrument for it anyway?
