Sneak Attack
[...] Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or ranged weapon.
You don't need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn't incapacitated, and you don't have disadvantage on the attack roll.
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Volley
You can use your action to make a ranged attack against any number of creatures within 10 feet of a point you can see within your weapon's range. You must have ammunition for each target, as normal, and you make a separate attack roll for each target.
You can only Sneak Attack once per turn. With you volley, you would either require advantage on the attack OR another enemy of the target standing 5ft next to it to apply Sneak Attack.
So, you can combine it, if the conditions of Sneak Attack are fulfilled, but only on one target!