Got new iPhone 4 back from Apple - but voice volume is down - how does one increase the volume?

Got new iPhone 4 back from Apple - but voice volume is down - how does one increase the volume?

I have tried the usual buttons on side of phone.

I removed volume lock and set volume to maximum.

But still conversations are a strain to hear.

It's OK using the head set - sound is good, just not so good using internal speaker.

If there are any other ideas I can try, greatly appreciated, otherwise it goes back to Apple!

Dave