I've encountered sound problems in the past as well like occasional absence of sms notifications, muted unlock sound etc. I found that it is a software problem of sorts. Try reset the phone or do a restore. That should fix it.

That is weird... Is that really works?? 

@kmheing
When u press the volume button ... is there a 'headphone' word on top of the word 'Ringer'?

I know why don't have sound...sure you turn the button on ur left hand side of ur iphone that will switch into silent mode

go to settings --> sound --> and try selecting any ringtone. Got sound?

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Added on November 23, 2008, 5:35 pmDid u recently use winterboard to apply some theme?

I last time did encountered a problem like u..

I just plugged in and out with my earphone then the problem get resolved.

It seems like ur speaker is working, maybe software or settings failure?

I have that problem sometimes, i just reboot or respring and tadaaa .. sounds is back on ..

Or just opening iTunes and play any songs inside will help too sometimes ..

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Added on November 24, 2008, 12:08 am

He was saying that out of sarcasm ..

TS, since I'm not with 2.1FW, I may be wrong but it's not hurt to give it a try. How about changing another theme or use the default theme to rectify whether the sound problem is still the same.

A similar problem happens to my 2.2 got ringtones but when answer a phone call no sound have to put on speaker or using bluetooth.
When I press the volume button instead of ringer it appears as headphones ringer.
ipod speaker no sound but when insert earphone got sound, no clicking sound when pressing keyboard and no unlock sound too.

Solution: Tried insert the headphone jack many times still the same. on off phone still the same. Hard reset also still the same.

At last resort- reset all content and settings- it took almost 2 hours. JB again. At last the sound came back to normal.

It works with my phone it worth a try..