I'd use Ghost Sound as the base spell, if not have the character stumble across an odd little bit of gnomish engineering going for about a thousand gold.

The key point is that if it's just a little fluff item for the character, it shouldn't eat up too much WBL. A quick look at the magic item creation rules says that 2k base for a continuous item, x1/2 for being a cantrip, x1 for requiring a base first level caster, x4 for for being a continuous item when the base spell is 1 round/level. 4K honestly sounds a little high for me, so making it too bulky to move around easily (making it less useful as adventuring gear) should knock it down to a 1 or 2k piece of fluff.