My samsung mini can hear callers but not hear me. Also the speakerphone aint working too.

To be check handset speakerphone part or electronic part for that speakerphone. There is difference handset speakerphone and speaker.

Bad mic. Does it work with the handset? Maybe in headset mode?

No, that is not due to battery. Your mouthpiece or receiver's earpiece might be bad. To make sure, please record something and try playing it. If you can hear what you recorded, your mouthpiece is fully functional. Now test whether the receiver earpiece is not working. Tell the receiver to set his phone is speakerphone mode. If he can hear what you say in speakerphone mode, then the receiver's earpiece is not fully functional.
Hope this helps.

remove the battery for about ten minutes. put it back in and turn on the phone that should sort it out

your cell phone probably has an internal speaker for you to hold the phone up to your ear and talk and an external speaker that is used for the ringtone and for you to use the talk on speakerphone option. You can try to use the speakerphone to speakerphone and see if you can hear your callers.
If you phone's internal speaker is not working, the phone needs to be repaired or replaced.

Manual:
http://www.cellphones.ca/upload/manuals/samsung-sph-m300-manual.pdf

From Page 39 of the manual:


Selecting Options (right softkey) during a call displays a list of
features you may use during the course of a call. To select an
option, press the corresponding keypad number or highlight
the option and press . The following options are available
through the Options menu:
 (1) Key Mute/Unmute allows you to mute the microphone
on your phone so the caller is unable to hear any sound.
 (2) Turn Speaker On enables the speakerphone feature.
 (3) Set Whisper increases the microphone to its maximum
level. During a call turn on Set Whisper when speaking
softly or when the caller cannot hear your voice clearly.
This mode is useful in meetings or public places.
 (4) Call History checks your call log for Outgoing, Incoming,
Missed, and Recent calls. You can also erase the logs from
this menu.
 (5) Contacts accesses the Contacts menu.
 (6) 3-Way Call allows you to talk to two different people at
the same time. (For more information, see “Making a
Three-Way Call” on page 189.)
 (7) Messaging accesses the Messaging menu options.
 (8) Voice Memo allows you to record incoming audio from
your conversation. (For more information, see “Managing
Voice Memos” on page 135.)
 (9) Tools accesses the Tools menu options.

Manual:

It means your voice IC is fine and working you have to change mic of the phone.

Its a small speaker upper side of the display under the casing.

have the earpiece replaced

no you need to have internal speaker replaced. You can get the part on ebay