I have managed to solve the sound card issue: I ran #make menuconfig entered the section "device drivers", then marked sound card support as M, alsa as M and PCI sound card as * and the modules for my card as M and voilá now there a kernel driver in use: snd_oxigen

03:04.0 Multimedia audio controller: C-Media Electronics Inc CMI8788 [Oxygen HD
Audio]
Subsystem: ASUSTeK Computer Inc. CMI8788 [Oxygen HD Audio]
Flags: bus master, medium devsel, latency 64, IRQ 16
I/O ports at d800 [size=256]
Capabilities: [c0] Power Management version 2
Kernel driver in use: snd_oxygen
Kernel modules: snd_oxygen

after that, then with the help of the IRC (tired890) I have created the .asoundrc file on my ~ directory because the sound card was not the default device

bash-4.3# cat /proc/asound/cards
0 [DGX ]: CMI8786 - Xonar DGX
C-Media Oxygen HD Audio at 0xd800, irq 16
1 [NVidia ]: HDA-Intel - HDA NVidia
HDA NVidia at 0xfea7c000 irq 19
bash-4.3#

pcm.!default {
type hw
card 1
}
ctl.!default {
type hw
card 1
}