Arcane Accompaniment( Player's Handbook II, p. 74)
You infuse your performance with magical energy, allowing its effects to continue even as you attend to other tasks.
As a swift action, you can expend a prepared spell or a spell slot to extend the duration of your bardic music ability after you stop performing. You extend the duration a number of rounds equal to the level of the spell used in this manner. This extension is in addition to the normal duration of the effect after you stop your performance. You can expend only one spell slot to extend the duration of your bardic music. The slot can come from any of your arcane caster classes, not just bard. This has no effect on bardic music or similar abilities with a duration of instantaneous or permanent.