Audio-editor (e.g., Audacity) is whatever PC/laptop software program you used to edit, splice, modify, and save the .MP3 sound file. So should be as simple as opening the .MP3, select the entire waveform, decrease by 6dB or whatever, re-save.
Series resistor is probably faster. If 8 ohm speaker, adding 8 ohm resistor drops power 6dB. Not as efficient as half your electrical power is wasted as heat in the series resistor but as this is not a battery application it should not be a big deal.
As mentioned earlier, with these highly integrated digital chips (as used on the MP3 module), it may not be as simple as applying a long pulse to the volume button(s). As alluded to in my previous post, these chips tend to share pins between multiple function including changing a pin from being an input to an output hundreds of times per second. This is done to reduce the pin-count and footprint of the IC. In other words, the VOL+ and VOL- control lines may be used for other functions so directly driving from the Arduino may conflict with chip operation unless you only drive the pins during the magical intervals when those pins are acting as VOL+ and VOL-. Again, I don't know if this specific MP3 player module uses this so-called "pin multiplexing". But note that the magic MP3 chips has only 16-pins on it...and it is talking to the SD card, can talk USB to a PC over the micro-USB connector, is blinking a LED, is possibly performing some kind of battery charging, generating stereo audio, listening to 4 buttons, etc.. you have to wonder how it can do this with "only" 16-pins!
