I have experienced a similar problem to this. Sometimes when I use the Voice Search app to "Call <name> mobile" I will hear them just fine, but they never hear me. However, I'm usually always using the phone's earpiece and not the speaker when this occurs. It doesn't happen every time, and it hasn't happened in a while so it may have been resolved in a software/app update that I missed. Of course, I only occasionally use this so I may have just lucked out lately.

My most recent problem (and the reason I was brought to this thread) is that my external speaker died last night, as everything is now poppy and distorted. I'm afraid the only fix to that is a replacement phone.

Edit: Oh, and I forgot to mention this but I'm running the leaked DK28, so I could be experiencing a number of bugs that may not apply to those running stock 2.1.