Step 1 - Nature of sound waves

Sound is a mechanical, longitudinal wave. It travels by making the particles of the medium vibrate, forming compressions and rarefactions.

Step 2 - Condition needed

A material medium (solid, liquid or gas) is necessary.

Step 3 - In vacuum

A vacuum has no particles, so there is nothing to vibrate.

Answer: Vacuum

(This is why no sound can be heard in outer space, though light reaches us easily since light needs no medium.)