Metamagic Song( Races of Stone, p. 142)
You can channel the power of your bardic music into your magic, allowing you to pay the cost of metamagic feats by spending uses of your bardic music ability.
Any two metamagic feats, bardic music,
When you spontaneously cast an arcane spell that is enhanced by a metamagic feat, you can spend a number of uses of your bardic music ability equal to the number of extra levels that the metamagic feat imposes on the spell rather than raising the spell's effective level. Applying metamagic spontaneously using this method increases the casting time of the spell normally. You cannot use the Metamagic Song feat to add metamagic feats that would make the spell's effective level higher than the highest level of spell that you can cast normally.
When applying a metamagic feat to a spell, you must either use Metamagic Song to completely offset the increase in the spell's effective level or apply the metamagic feat normally and cast the spell at its higher level. You cannot partially increase the level of the spell (or "split" the cost) to reduce the number of uses of bardic music spent when using the Metamagic Song feat. Only one method can be used to pay for metamagic feats that affect a single spell. You cannot use the Metamagic Song feat to improve a spell with the Silent Spell metamagic feat.