Found my old 3DS XL 1st Gen Super Smash Bros but I have a sound problem

So… Glad I found my old 3DS XL. Battery was completely dead but I had no problem to charge it.
Everything is looking great but I have an issue with the sound. The console delivers a mid volume sound and the sound slider doesn't seem to work.
Same thing with wired earphone plugged.
And the weird thing is the sound looses volume if I use the upper screen 3D slide. The more I push the 3D effect, the more the volume fade.
The speakers are fine IMHO because the sound while taking a picture is louder.
Any suggestion ?
Thank you.