well Im not connecting this (the final project) to small speakr but normal speakers, even pre-amp concert speakers so dont know then if I should go with your recomendation??
if I understand what you are doing, a mixer really isn't what you want. And the regular op-amps don't have enough power (Watts & current) to drive a 4 or 8 ohm speaker.
A mixer is for mixing sounds... i.e. With a 3-input mixer, you could blend two guitars and a microphone (assuming they've been pre-amplified). You can drive a preamp directly from the Arduino. You just need a series capacitor
(maybe 1uF) to knock-out the DC component. A volume control (pot) would be a good idea too. A series resistor (maybe 1K) wouldn't hurt either. It will limit the current so that you don't wipe-out the Arduino if you accidently short-out the output (or if you accidently connect a speaker).If you want to directly drive a speader, National Semiconductor makes lots of "Power Amplifier" chips.
Some of these can operate from 5V, and the circuit will usually be simpler than the mixer.
There are limits to how much power you can get from a given power supply:
With a 5V supply, you can theoretically get a little more than 1/3rd of a Watt into 8 Ohms.
With 12V, you can theoretically get a little more than 2W.
With +/- 12V, you get 4 times the power (8-9W).
With 4 Ohms, you double the power.
With a bridge* amplifier, you get 4 times the power. (i.e. You should be able to get more than 2W with a 5V bridge amplifier into 4 ohms.)
In the real world, the amplifier can't put the full power supply voltage across the load and you get a little less power than you calculate.
* A bridge amplifier is a push-pull design with two "hot" wires going to the speaker, instead of one side being connected to ground.