Also, if I use an amplifier with more Watt output, the buzzing/humming will we reduced too, right?
7.4V peak-to-peak is 3.7 peak. Myltiply that by 0.707 to get the RMS (assuming a sine wave). That's 2.6V RMS across an 8 Ohm speaker, and that works-out to about 0.85W maximum. You can double that by using a 4-Ohm speaker.
...regulated or filtered power supply will help to reduce the noise
Nevertheless I could increase the audio quality* by adding a capacitor between + and -
Meanwhile I will try to figure out where the clicking noise at the end of the sound file is comming from