Audio is cracking on Asus VivoBook S14 (device [8086:9d71] (rev 21))

Device: Asus VivoBook S14
Ubuntu version used: 18.10
Linux version: 4.18.0-15-generic #16-Ubuntu SMP Thu Feb 7 10:56:39 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

Summary
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Audio is cracking when playing sounds (using Videos, VLC, Firefox with Vimeo and Youtube) on both internal speakers and when using HDMI audio, with the laptop connected to an external monitor.

The fix described on the Ubuntu Wiki [1] helped solving the issue, but as stated in the link, I report an issue so audio can work out of the box for this device.

To fix audio on this device, I added

options snd-hda-intel position_fix=2

in /etc/modprobe.d/alsa-base.conf.

Steps to reproduce
------------------

1. Install Ubuntu 18.10 on the device
2. Make sure audio output is set to "internal speakers", and that over-amplification is OFF.
3. Start playing a sound (easier to reproduce when listening to a movie trailer) using either Videos, VLC or Firefox.
4. (optional) Connect an external display with audio output using HDMI cable
5. Make sure the audio output is set to use HDMI audio output in the sounds properties
6. Repeat step 3

In both steps 3 and 6, the sound is cracking, even when not set to the maximum volume.

Audio information (from lspci):

00:1f.3 Audio device [0403]: Intel Corporation Sunrise Point-LP HD Audio [8086:9d71] (rev 21)
 Subsystem: ASUSTeK Computer Inc. Sunrise Point-LP HD Audio [1043:1c50]
 Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+
 Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
 Latency: 32, Cache Line Size: 64 bytes
 Interrupt: pin A routed to IRQ 131
 Region 0: Memory at ef428000 (64-bit, non-prefetchable) [size=16K]
 Region 4: Memory at ef400000 (64-bit, non-prefetchable) [size=64K]
 Capabilities: [50] Power Management version 3
  Flags: PMEClk- DSI- D1- D2- AuxCurrent=55mA PME(D0-,D1-,D2-,D3hot+,D3cold+)
  Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME-
 Capabilities: [60] MSI: Enable+ Count=1/1 Maskable- 64bit+
  Address: 00000000fee00398 Data: 0000
 Kernel driver in use: snd_hda_intel
 Kernel modules: snd_hda_intel, snd_soc_skl

I attached an sosreport archive with all the information about this device. Please check in the archive's `/sosreport-20190215151233/sos_commands/` for more logs (alsa-related logs are in `/sosreport-20190215151233/sos_commands/soundcard`, for instance).

P.S.: I could not create an issue using `ubuntu-bug -s audio` possibly because I had already modified the alsa-base.conf file, and therefore ubuntu-bug told me the system has been modified (or something along these lines). Sorry about that!

[1] https://wiki.ubuntu.com/Audio/PositionReporting