OK, you've already been informed the hole centered under the keypad serves a great purpose!
Regarding your other issue, I'm in agreement with Smeg. Either something is clogging the mic hole - or, maybe you got a bum phone. My phone is a twin to yours and the mic hole is in the exact same place but it is adequately sensitive (quite so IMHO).
When my phone was new to me and I didn't have it figured out yet how to use the BT headset with it, I made the mistake of answering an incoming call using the green call button on the phone. I did this while my phone was at arms length away from my face. And, though I could not hear my caller, they could hear me. Actually, I eventually did hear my caller's tiny little voice yelling at me through the phone telling me they could hear me! I finally figured out that if you answer the call on your phone...you answer the call on your phone EVEN IF you have your BT earpiece on.
My point is, the phone was nowhere near my mouth and my callers could hear me.
Have you tried using the voice notes app and recording your voice at different distances from your phone? For example, the first recording would be with the phone held to your ear like you're placing a call. Then try a recording with the phone on a table top in front of you like you're using the speakerphone. And lastly, maybe about 3 feet away like it might be if you had several people around a desk for a conference call.
I just did those three recordings on my phone and all are usable. The last one is pushing it...but, it's still usable. I spoke at a normal loudness for all three. From my little test just now, I now know I would have to speak quite a bit louder if it were at "conference call" distance. I just made one more recording at that distance and spoke louder...it worked great!
I'm assuming that since my phone "records" my voice fine that it translates the mic is working as it should and a person on the other end of a call would hear me the same as how my recordings sound. I hope that's a safe assumption.
Try it and see if you're phone has the same problem with recording as it does with calls. Or, is it just during calls the mic seems to not work right? I'd also have someone else use my phone and call me so I could hear what they sound like just so I could hear it with my own ears. Or, when I get a new headset I call my house and let the answer machine pick up then I leave a message. That way I KNOW for certain what it sounds like to people on the other end of a phone call talking to me. I originally tried asking other people how it sounded and realized it's all so subjective and that I need to hear it with my own ears and decide for myself how it sounds.
OH...one more thing...I just re-read your post and you say that using an earpiece makes things even worse. This really leads me to believe there is something wrong with your particular phone. Using the wired headset that came with my phone, or either of two different BT headsets that I use...the sound is terrific on mine. It is VERY LOUD.
Without a headset plugged into your phone, what profile does your phone say it's using? If it says "headset" even though you do NOT have a headset plugged in - there's a problem with the headset jack hole. I've read fixes that suggested using canned air to blow the hole out and/or quickly inserting and pulling the headset plug in and out several times to dislodge any foreign matter that might be stuck in there. At any rate, the profile should be "normal" if you're not using a wired or BT headset at the moment.
Devices: T-Mobile MDA (Cingular SIM), i-mate SP5m (sitting in a drawer at the moment while I attempt to migrate to PPC permanently), Cingular 3125 (unlocked and in use on T-Mobile), Motorola MPx220 (retired), Motorola MPx200 (retired)
Accessories: Plantronics Explorer 330 & 2G Patriot Micro-SD (use w/MDA), Kingston 2G mini-SD used in SP5m, Plantronics Discovery 640 & 2G San Disk micro-SD (use w/3125), Jabra A210 BT adapter + Motorola HS805 (retired)