Internal microphone doesn't work with Realtek ALC268 chipset

I have this problem with Ubuntu 18.04, 18.10 and Linux Mint 19, on my HP Pavilion dv6700, with a Realtek ALC268 chipset.
Audio is working but not internal and external microphones.

As explained in https://elubuntero.wordpress.com/tag/realtek-alc268/ , I tried to add linux-backports-modules-*** but this package isn't anymore in Ubuntu and Mint repositories.

I tried all options as explained in https://doc.ubuntu-fr.org/audio_intel_hda, without result.

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: pulseaudio 1:12.2-0ubuntu4
ProcVersionSignature: Ubuntu 4.18.0-10.11-generic 4.18.12
Uname: Linux 4.18.0-10-generic x86_64
ApportVersion: 2.20.10-0ubuntu13
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: ubuntu 6306 F.... pulseaudio
CasperVersion: 1.399
Date: Wed Dec 19 09:35:53 2018
LiveMediaBuild: Ubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.3)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
PulseList: Error: command ['pacmd', 'list'] failed with exit code 1: No PulseAudio daemon running, or not running as session daemon.
SourcePackage: pulseaudio
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 06/16/2008
dmi.bios.vendor: Hewlett-Packard
dmi.bios.version: F.58
dmi.board.name: 30D2
dmi.board.vendor: Quanta
dmi.board.version: 79.2E
dmi.chassis.type: 10
dmi.chassis.vendor: Quanta
dmi.chassis.version: N/A
dmi.modalias: dmi:bvnHewlett-Packard:bvrF.58:bd06/16/2008:svnHewlett-Packard:pnHPPaviliondv6700NotebookPC:pvrRev1:rvnQuanta:rn30D2:rvr79.2E:cvnQuanta:ct10:cvrN/A:
dmi.product.family: 103C_5335KV
dmi.product.name: HP Pavilion dv6700 Notebook PC
dmi.product.sku: KS356EA#ABF
dmi.product.version: Rev 1
dmi.sys.vendor: Hewlett-Packard