I've had this issue several times as well. After figuring out what was happening (mass storage mode), I moved the ringtones/notifications sounds to the system directory.
That worked... for a bit. Then it happened again.
Strange thing is, after moving them into the system, I deleted them from the SD(where they were at first) so I could make sure the idea worked. And it did.
But since then, I have tried sticking with the stock ringtones that came with the ROM, and it will still change them sometimes after using mass storage. And it did this before root with the stock LG ROM too... is this a known bug with the Esteem, or is it just my phone?