If you're using a USB mic, tough luck you won't be able to do it with vista, maybe with a thirth party program.
If you're using a mic that is connected to your sound card,
go to the same as you are in your screen above
playback -> Speaker / Headphone (or which ever device is enabled) -> advanced -> Levels -> and simply flop your sound card entry to the highest possible (if it's to loud you make it less hard, but that's quite easy to understand)
I'm using a dutch version of Vista so i might get some words wrong.
In addition to the above, double click on Microphone in your image above to open it's properties page. See if you may have the option below. If you do, then check the box below in red. It should then playback through your speakers.
I'm having this exact same problem. No option for Listen listed under my microphone's properties. Please, someone help, I would really like to be able to listen to myself through my laptop speakers (Toshiba L305-S5894, Realtek HD Audio)
I tried it that way, but still no sound. It appears that microphone playback is intentionally disabled by Realtek, per Toshiba's instructions, but apparently only on laptops running Vista x32 with HD Audio. I just don't see why they would do that; I've never seen the mic playback disabled on any other soundcard, computer, or Windows OS. It's weird. I've appealed to Toshiba and Realtek, but if the posts I've read on other forums across the net are any indication, Toshiba and Realtek do not see this as a problem, and will not provide a solution.
Thank you for your suggestion. I appreciate your input.
 --> Right Click the Volume Icon at the bottom right corner of your screen [The Task bar].
 --> Click [Playback Devices]
 --> Right Click the [Speakers] Icon and click Properties
 --> Click the [Levels] Tab; and there you will see the Microphone Volume slider (The one at the very bottom of the "HD Audio output" and "PC Beep" sliders where you can un-mute the mic playback (By clicking the unmute icon, than slide the volume up to where you prefer it... Simple I hope I was able to help someone with this...