In D&D 3.5, as a bard I have to make concentration checks to maintain some of the Bardic Music abilities. What's the DC of those concentration checks?
These abilities don't "require concentration checks". The bard entry just says they "require concentration" - meaning that you must spend a standard action each round to keep them going.
Also, according to the concentration entry, unless specified otherwise, you only make concentration checks when some specific distraction (typically damage) might make concentration difficult.
There's No DC Until You're Interrupted...
When an effect requires concentration, the character creating the effect can just say, "I'm concentrating on the effect," and the concentration happens, no roll required. This is explained in the section describing how to read a spell's Duration: A spell with a duration of concentration
lasts as long as you concentrate on it. Concentrating to maintain a spell is a standard action that does not provoke attacks of opportunity. Anything that could break your concentration when casting a spell can also break your concentration while you’re maintaining one, causing the spell to end.
You can’t cast a spell while concentrating on another one. Sometimes a spell lasts for a short time after you cease concentrating.
Emphasis mine, and, as you've noted, that's kind of important. (While the above text is for spells specifically, spell-like and supernatural abilities use the same rules except when noted.)
...Then You Must Roll
Thus it's possible to disrupt a bard's concentration on his bardic music just like it's possible to disrupt a wizard's concentration when he's either attempting to cast a spell or already concentrating on a spell. How this disruption can occur is in the section on casting spells, appropriately called Concentration. It's a hefty list, and I won't to reproduce here; suffice it to say, a lot of stuff can disrupt concentration, chief among them enemy weapons in vital areas.
Determining Spell-equivalents and Spell Levels of Spell-like and Supernatural Abilities
If something does disrupt the bard's concentration while he's using a bardic music ability that requires concentration, the bard makes a Concentration skill check. Unfortunately, the game omits the spell levels of the bard's abilities, forcing the DM to determine the equivalent spell level--and, subsequently, the Concentration check DCs--by using the rules for spell-like abilities and supernatural abilities. Suggestions appear below.
- The bardic music ability fascinate has no equivalent spell, although it's similar to the 1st-level Sor/Wiz spell hypnotism [ench] (PH 242-3). Ask the DM. Note: I recommend it be treated as a 1st-level spell.
- The bardic music ability inspire competence has no equivalent spell. Ask the DM. Note: I recommend it being treated as a 2nd-level spell.
- The bardic music ability song of freedom is equivalent to the 5th-level Sor/Wiz spell break enchantment [abjur] (PH 207), making this the equivalent of a 5th-level spell.