You can steal some monster abilities that function like bardic music
This answer ranks very high on the cheese ladder, but there are some monster special abilities that you can steal in order to get more bardic music uses. A lot of the time, though, having a monster work for you will be stronger than stealing its abilities. So this can sometimes be considered a less-cheesy application of spells like planar binding or simulacrum.
How to steal abilities
Two good ways of stealing abilities are: channeling a celestial, or using the ability rip spell.
Ability rip spell
This 7th-level spell from Serpent Kingdoms allows the caster to transfer a supernatural ability from one creature to another, with a 1-hour casting time and an hour/level duration. The catch is that the receiving creature must permanently give up one of its own supernatural abilities (or its Hit Dice or Constitution, if it has no supernatural abilities of its own).
There are two ways to get around this:
- Give up a temporary supernatural ability:
- You can gain one by using a spell from the polymorph subschool (the lowest level example is probably dragonshape, least, from Dragon Magic). You can't normally cast spells while affected by one of these spells, so someone else is going to have to cast Ability Rip for you.
- Some martial maneuvers from the Tome of Battle are supernatural abilities, such as Cloak of Deception or Flame's Blessing. You can cast Heroics to gain the Martial Study feat in order to temporarily gain access to one of these supernatural maneuvers.
- When binding a vestige, you can gain temporary supernatural abilities. See the Expunge the Supernatural spell (Tome of Magic) as a precedent for this strategy.
- Ability rip says that the spell is lost "permanently". This could be taken to mean that the effect that removes the ability has a permanent duration (instead of an instantaneous one). In this case, dispel magic should be able to get rid of it, thus restoring the lost ability.
Channeling a celestial
Chapter 2 of the Book of Exalted Deeds offers rules for a mortal to channel a celestial. Long story short, this allows you to do a few pretty strong things, not least of which is the ability to use the celestial's spell-like and supernatural abilities.
In order for a celestial to be channeled, it must either have the channeling spell-like ability, or the mortal must cast the spells channel celestial (7th level, limited to celestials of up to 12 HD) and channel celestial, greater (9th level, limited to celestials of up to 24 HD). There are ways to steal the Channeling ability from one celestial in order to give it to another one, but this answer will assume that the spells will be used.
Monsters to steal from
The most interesting monsters to steal from are chaotic good outsiders. In particular, these three creatures stand out: the firre, the lillend and the members of the powerful Court of Stars.
Only the firre and the lilend are available as races to a player character, and unfortunately their level adjustment is way too high for it to be worth taking them for the purposes of bardic music uses. So stealing their abilities is the way to go here.
Firre (Book of Exalted Deeds)
This eladrin gets the song supernatural ability, which allows it to use the inspire courage, inspire competence and suggestion bardic music effects an unlimited number of times per day. However, it is limited to those effects only, and it doesn't really gain any "uses" of bardic music for the purposes of feats like Metamagic Song. It has 8 HD and thus can be called using the normal version of the planar binding spell.
This strategy is going to take a 7th level spell and a 6th level spell every day. As a side note, you can easily cast Shapechange to transform into a Firre and gain the song ability. This is not possible with the following two examples.
Lillend (Monster Manual)
This celestial gets bardic music (of a 6th level bard) as a spell-like ability. That means the easiest way to steal it would be to channel it. Another alternative is to have a Lillend cohort and force them to take the Supernatural Transformation feat (Savage Species). This can be used to make their bardic music ability a supernatural ability, allowing us to steal it with ability rip.
It has 7 HD and thus can be called using the normal version of the planar binding spell. As such, strategy is going to take a 7th spell and a 6th level spell every day.
The Court of Stars (Book of Exalted Deeds)
This is a series of 3 unique celestials that rule over all the eladrin, and each gets bardic music (of a 20th level bard) as a supernatural ability. They all have more than 24 HD, so we cannot channel them using channel celestial, greater. However, we can create duplicates of them using the simulacrum spell. These duplicates have less HD and can be channeled. We can also easily steal the ability using Ability Rip.
Creating a simulacrum has an XP and a gold cost, but overall its more convenient that summoning a celestial every day with planar binding. Thus, this strategy can also be used for the firre and the lillend. The spell does require a material component of "rough snow or ice form, and some piece of the creature to be duplicated (hair, nail, or the like) must be placed inside the snow or ice." This component technically has no cost, so it could conceivably be inside a spell component pouch or be ignored with the Eschew Materials feat.
One thing your DM must decide is how the abilities are supposed to stack. Normally, a creature's innate bardic music ability stacks with their bard levels for the purposes of determining what bardic music effects they have access to. So a bard who stole bardic music from both a member of the Court of Stars and a lillend could theoretically have 26th level bardic music, on top of his bardic music from normal levels.
Your DM could rule that each stolen ability creates a separate instance of bardic music however, in which case you would not be able to, for example, use inspire greatness with the 6 uses granted by the lillend version of the ability.