How can I tell linux to send directly from my microphone to my loudspeaker?
This depends on your audio system.
By default, your mic input should be already send to your speaker.
Please specify your problem a little bit more in detail.
A common problem is, that you will have to unmute your mic channel.
The major audio systems for linux since 2.6 is ALSA.
The default mixer application is curses based and can be opened on the terminal with
If you are using an older installation, maybe your audio system is OSS. The default mixing application for this one is