I also have a very similar problem with my Alienware 18x r2 Running Linux Mint 17.2   

I just installed the latest " Kernel 4.1.7-040107-generic " hoping it would help but no changes

HDA  Intel
HDA Nvida
Realtek ALC3661

My problem is my sound was only coming through my Sub 
So sound is very Dull and Bad Quallity
I have tried HDAJackRetask and have sound coming from the two normal speakers  but not the Sub

I contacted Dell they don’t really seem interested 

Sound Card Specs

Realtek ACL3661

Advanced Linux Sound Architecture Driver Version k3.16.0-38-generic.


List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC3661 Analog [ALC3661 Analog]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 3: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 7: HDMI 1 [HDMI 1]
Subdevices: 1/1
Subdevice #0: subdevice #0


gary@gary-Alienware-18 ~ $ lsmod | grep snd
snd_hda_codec_hdmi 47548 1
snd_hda_codec_realtek 77467 1
snd_hda_codec_generic 69011 1 snd_hda_codec_realtek
snd_hda_intel 30469 7
snd_hda_controller 30228 1 snd_hda_intel
snd_hda_codec 139682 5 snd_hda_codec_realtek,snd_hda_codec_hdmi,snd_hda_codec_generic,snd_hda_intel,snd_hda_controller
snd_hwdep 17698 1 snd_hda_codec
snd_pcm 104112 5 snd_hda_codec_hdmi,snd_hda_codec,snd_hda_intel,snd_hda_controller
snd_seq_midi 13564 0
snd_seq_midi_event 14899 1 snd_seq_midi
snd_rawmidi 30876 1 snd_seq_midi
snd_seq 63074 2 snd_seq_midi_event,snd_seq_midi
snd_seq_device 14497 3 snd_seq,snd_rawmidi,snd_seq_midi
snd_timer 29562 2 snd_pcm,snd_seq
snd 79468 24 snd_hda_codec_realtek,snd_hwdep,snd_timer,snd_hda_codec_hdmi,snd_pcm,snd_seq,snd_rawmidi,snd_hda_codec_generic,snd_hda_codec,snd_hda_intel,snd_seq_device
soundcore 15047 2 snd,snd_hda_codec


ry@gary-Alienware-18 ~ $ cat /proc/asound/devices
1: : sequencer
2: [ 0] : control
3: [ 0- 0]: digital audio playback
4: [ 0- 0]: digital audio capture
5: [ 0- 0]: hardware dependent
6: [ 1] : control
7: [ 1- 3]: digital audio playback
8: [ 1- 7]: digital audio playback
9: [ 1- 0]: hardware dependent
33: : timer

┌──────────────────────────── AlsaMixer v1.0.27.2 ─────────────────────────────┐
│ Card: HDA Intel PCH F1: Help │
│ Chip: Realtek ALC3661 F2: System information │
│ View: F3:[Playback] F4: Capture F5: All F6: Select sound card │
│ Item: Master [dB gain: -23.25] Esc: Exit │
│ │
│ ┌──┐ ┌──┐ ┌──┐ ┌──┐ ┌──┐ ┌──┐ │
│ │ │ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │ │ │
│ │ │ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │ │ │
│ │ │ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │ │ →
│ │ │ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │ │ →
│ │ │ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │ │ →
│ │ │ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │ │ →
│ │ │ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │ │ →
│ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │ │ →
│ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │ │ →
│ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │ │ │
│ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │▒▒│ │ │ │
│ ├──┤ ├──┤ ├──┤ ┌──┐ ├──┤ ┌──┐ └──┘ ├──┤ │
│ │OO│ │OO│ │OO│ │OO│ │OO│ │OO│ │MM│ │
│ └──┘ └──┘ └──┘ └──┘ └──┘ └──┘ └──┘ │
│ 36 100<>100 100<>100 100<>100 100<>100 0<>0 │
│ < Master >Headphon Headphon Speaker Speaker Speaker PCM Mic │
└────────────────────────────────────────────────────────────────────────────

I am hoping there is going to an easy Fix  
There are many others with Alienware Rigs waiting for Linux to be compatible
Cheers
Gaz