Try plugging in some headphones, and then removing them. I'm not sure the iPhone 3G has that bug, but the iPhone 2G definitely does. If you ever lose audio, just plug and remove some headphones.
Alex,
its a bug, I am having the same issue as well. Sometimes sound and sometime no. Sometime the txt will show and sometime it will not when i already have it set to NO PREVIEW it still shows.
I am having the exact same issue. My phone will vibrate when I have the sound on but when a text message comes in, I don't get the texting sound. I rebooted the phone and it worked fine for about 30 min. Now it is doing it again..no text sounds...im going to try the headphone thing now....
tried plugging in headphones and removing the headphones, still no texting sound..but all the other sounds work...this is definitely a bug..how can we get his escalated?
I did a full restore earlier and it is still doing it....i just did a restore all settings under general and now the text sound is back...but let's see how long it will last?