Did you open your SMS app first before clicking on "Menu > Settings" ?
If not, check there first.
And check the standard "menu > sounds & display" as well.
No idea what the diff is between voice call & caller ringtone - you'd think they'd be the same... maybe caller ringtone is to assign the sound to a specific contact only, vs the voice call - assign to all callers...
You're trying to assign the sound by long pressing on the droid file itself. To set it as a text ringtone, follow my suggestions above.
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