txt tone intermittantly not playing

does anyone else have a repeating problem where the txt message tone doesn't play?

sometimes the iphone will decide it will play the tone when a txt comes in, other times it doesn't. the on-screen alert still appears, but no tone.

all other tones work, push email tone plays fine, call tones play fine. just seem to be txt tone.

this happened on v2.0 and v2.01.
i've rebooted the phone and the problem sometimes fixes itself only to happen again later.