If music be the food of love, play on
Orsino is asking for more music because he is frustrated in his courtship of Countess Olivia. He muses that an excess of music might cure his obsession with love, in the way that eating too much removes one's appetite for food.
Music plays an important part in Shakespeare's plays and is often used to carry the plot. It's reasonable to surmise that he did believe it the be 'the food of love'.
From Shakespeare's Twelfth Night, 1602:
If music be the food of love, play on;
Give me excess of it, that, surfeiting,
The appetite may sicken, and so die.
That strain again! it had a dying fall:
O, it came o'er my ear like the sweet sound,
That breathes upon a bank of violets,
Stealing and giving odour! Enough; no more:
'Tis not so sweet now as it was before.
O spirit of love! how quick and fresh art thou,
That, notwithstanding thy capacity
Receiveth as the sea, nought enters there,
Of what validity and pitch soe'er,
But falls into abatement and low price,
Even in a minute: so full of shapes is fancy
That it alone is high fantastical.
See other phrases and sayings from Shakespeare.