I bought my N900 from a Nokia flagship store only 3 months ago, but now when I try to call someone they can not hear me. I can use it with a headset only. Is there any way to fix it?

Sounds like you have a problem with the microphone.
Make a backup of everything you have on the phone and take it to your nearest service centre for repair.

Yes, I have already done that. Thanks a lot.

I've similar problem. It seems that sometime N900 are not aware when I unplug the headphone, thus the speaker remain mute all the time. I think the springer in the socket does not return to its normal position to give correct sensor value, so I replug and unplug the headphone several times until it fixed.

It's quite annoying. Therefore I'd use bluetooth headphone instead when possible.

yah, very reasonable analysis

I seems 'lshal -m' might be of some help here as well.
To see if the headphones are recognised, in X Terminal type:
Plug in headset, you should have an output something like this:
14:01:32.876: platform_headphone property button.state.value = true
14:01:32.889: platform_headphone condition ButtonPressed = connection
14:01:34.034: platform_soc_audio_logicaldev_input property input.jack.type = {'headphone', 'microphone'}
14:01:34.134: platform_soc_audio_logicaldev_input property input.jack.type = {'headphone', 'microphone'}
14:01:34.218: platform_soc_audio_logicaldev_input property button.state.value = true
14:01:34.236: platform_soc_audio_logicaldev_input condition ButtonPressed = jack_insert
14:01:34.470: computer_logicaldev_input_1 added

Unplug headset:
14:02:06.923: platform_headphone property button.state.value = false
14:02:06.945: platform_headphone condition ButtonPressed = connection
14:02:07.246: computer_logicaldev_input_1 removed
14:02:07.258: platform_soc_audio_logicaldev_input property input.jack.type = {}
14:02:07.461: platform_soc_audio_logicaldev_input property input.jack.type = {}
14:02:07.606: platform_soc_audio_logicaldev_input property button.state.value = false
14:02:07.622: platform_soc_audio_logicaldev_input condition ButtonPressed = jack_insert

Plug headphones:
14:24:55.623: platform_headphone property button.state.value = true
14:24:55.637: platform_headphone condition ButtonPressed = connection
14:24:56.305: platform_soc_audio_logicaldev_input property input.jack.type = {'headphone'}
14:24:56.344: platform_soc_audio_logicaldev_input property button.state.value = true
14:24:56.367: platform_soc_audio_logicaldev_input condition ButtonPressed = jack_insert

Unplug headphones:
14:25:00.108: platform_soc_audio_logicaldev_input property input.jack.type = {}
14:25:00.117: platform_soc_audio_logicaldev_input property button.state.value = false
14:25:00.203: platform_soc_audio_logicaldev_input condition ButtonPressed = jack_insert
14:24:59.794: platform_headphone property button.state.value = false
14:24:59.804: platform_headphone condition ButtonPressed = connection

Plug in TV:
14:14:51.175: platform_headphone property button.state.value = true
14:14:51.185: platform_headphone condition ButtonPressed = connection
14:14:51.953: platform_soc_audio_logicaldev_input property input.jack.type = {'line-out', 'video-out'}
14:14:52.006: platform_soc_audio_logicaldev_input property input.jack.type = {'line-out', 'video-out'}
14:14:52.058: platform_soc_audio_logicaldev_input property button.state.value = true
14:14:52.065: platform_soc_audio_logicaldev_input condition ButtonPressed = jack_insert

Unplug TV:
14:15:00.918: platform_headphone property button.state.value = false
14:15:00.928: platform_headphone condition ButtonPressed = connection
14:15:01.240: platform_soc_audio_logicaldev_input property input.jack.type = {}
14:15:01.258: platform_soc_audio_logicaldev_input property input.jack.type = {}
14:15:01.361: platform_soc_audio_logicaldev_input property button.state.value = false
14:15:01.368: platform_soc_audio_logicaldev_input condition ButtonPressed = jack_insert

Plug in TV cable but no TV:
14:14:04.654: platform_headphone property button.state.value = true
14:14:04.668: platform_headphone condition ButtonPressed = connection
14:14:05.492: platform_soc_audio_logicaldev_input property button.state.value = true
14:14:05.516: platform_soc_audio_logicaldev_input condition ButtonPressed = jack_insert

Unplug TV cable with no TV:
14:14:34.151: platform_headphone property button.state.value = false
14:14:34.161: platform_headphone condition ButtonPressed = connection
14:14:34.417: platform_soc_audio_logicaldev_input property button.state.value = false
14:14:34.424: platform_soc_audio_logicaldev_input condition ButtonPressed = jack_insert