Sound Design vs Sound Mixing
Two different crafts, routinely quoted as one line item. Knowing the difference changes what you ask for.
These terms get used interchangeably, including by people selling them. They describe genuinely different work.
Sound design creates
Sound design is the making of material that does not exist in the recording. The wind through a street that was silent on the day. The mechanical texture of a machine that was added in VFX. The low tone under a scene that makes an ordinary conversation feel wrong.
A sound designer asks: what should this world sound like?
Sound mixing balances
Mixing takes everything — dialogue, music, effects, ambience — and decides, moment by moment, what the audience hears and how loudly.
A mixer asks: what should the audience be listening to right now?
Why the distinction matters to you
If you ask for a mix when you needed design, you get a well-balanced version of thin material. Everything is audible and the scene still feels empty.
If you ask for design when you needed a mix, you get rich material that fights itself, and dialogue that disappears under it.
Most projects need both
They are usually done by the same team, which is part of why the terms blur. But when you are reading a quote, it is worth knowing which one you are being charged for — "sound design" on a line item sometimes means a full build and sometimes means a library effect dropped in and balanced.
Ask what is included. A studio that does this work properly will have a clear answer.