I did so in my first music box a few years ago, a gift for my wife, thinking it might give a marginal 'spatial' effect. And I had a case large enough. But I can't say it's ever had any methodical testing. 
Also for projects in which you might want to project the sound in two directions; Halloween comes to mind.
And possibly deploying any very low impedance speakers in the junk box.
Speakers in series or parallel interfere constructively or destructively with each other, depending on the speaker enclosure, frequency content of the audio, the distances between speakers and the wiring polarity.
There definitely is a spatial effect, but with so many variables it is very difficult to predict the effect of a given build on what a person might hear.
In fixed-frequency applications, beamforming with speaker or transducer arrays can be very useful:
My input is that only genuine DF Robot Mini players work well. All copies I have tried all have problems! You can buy from PiHut the genuine DF Robot player (with the blue LED.) I have done lots of projects, but only use the genuine players which have given me no issues.
I already bought another cheap one, so I'll see, if I still have the problem I'll go with an official one.
I posted a sketch earlier, and as I understand DFPlayer, it should give sound for left in DAC_L and right in DAC_R.
I wont do that in the end, just a single speaker from SPK_1/2
I tried on 3.3V and now it doesn't work at all, whereas in 5V it works (unstable). I found out that in 3.3V when I replug the SD card, the atmega328P reboot, whereas in doesn't when in 5V. I don't understand where 1 should put the .1uF capa?
The clones use a different chip and typically have a red LED whereas genuine have blue LED, Pihut have them at £5.70 (+ postage)
There is no pin of the Arduino in your circuit of post 13 connected to any input signal to activate any audio from the DFR Player.
And you have still not shown the actual sketch you are now using that you say does not work.
The pin A0 and A1 are the one I use as Tx and Rx (respectively).
this is the EXACT setup I was using at the time I posted this drawing. The code I used was the one on this web site https://wiki.dfrobot.com/dfplayer_mini_sku_dfr0299 with the library "DFRobotDFPlayerMini by DFRobot", with the Tx and Rx port changed according to the setup showed earlier.
In the end, I found the issue, the code of the site and the library linked to the DFPlayer didn't work (for me at least) so I re-coded it for my use and now it works fine. I just have an issue with the low power mode but it looks like it's quite common, I'm just unable to get it awake back. Also, I have some noise in the speaker in idle, even in low power mode, I'm thinking about adding a mosfet to shut completely when un-used. I'm open to suggestion as it doesn't look like it's the best idea.
I have some new stuff, I changed the micro SD card, and now it's more stable, I also switched to an original DFPlayer, but I didn't see any changes with, also the official program still doesn't work and show me a no sd card error. Now I have a new issue, the sound is "buzzing", sometimes at the start, some other times after a few seconds, and some other times it doesn't happen at all. I have some videos (or audio) but I don't know how to share them.
Side note : it doesn't work on 3.3v
It seems that is very very sensitive to the DC input of the circuit, and that it messes quit a lot with it. Now it's buzzing, and it reset the whole circuit even when nothing is plugged into my atmega328P reset PIN
Do you have a clean and stable voltage going to it? I don’t suppose you have an oscilloscope you can use to see the supply voltage? What capacitor are you using for the DFPlayer? I would go with 100uf electrolytic.
I was using a 47uf electrolytic, on the oscilloscope, the input voltage was fluctuating when the DFPlayer was buzzing. I used to plug my uart to usb (that I also use as power delivery) into my computer, and I switch to a smartphone charger, and it's way more stable, so I think there was an issue with current. I have to make further tests to see if it corrects every thing.
