Replicant

It works fine on all devices as far as I know. Please report what device you're using and I'll double check. Then we can think about opening a bug report.

That is for a samsung galaxy sIII, it seems you tested it.
During conversation, I tried to decrease the volume but even at minimum level, it is very loud.
Is there any other mean to decrease volume, or to check the sound level is correctly decreasing ? A numeric value for example ?

So the problem really here that the lowest volume is too high, not that you cannot decrease the volume. It should be possible to tune this by either looking at the audio module config files (if any) or by changing the values directly in the code.
Since this change may only be relevant for you, I doubt it's worth submitting it to Replicant. Or at least, it should go through CM first.

Apparently, one would have to change the settings in device/samsung/i9300/audio/audio_hw.h to reduce the volume, unless only the modem can change it, in which case there isn't much to do.

You are right : decrease the volume works but lowest level is to loud, but I can hardly distinguish the difference between highest and lowest level.
To explain my experience : when I receive a call, I do not have to use loudspeaker to hear the conversation, even when volume is set at lowest level, and people next to me can hear also.

I will look for the good variable, try a new one and test it. Thank you for your informations Paul.

• Settings -> Sound -> Volumes -> [X] Volume keys control ringtone volume (which should be disabled in your case)
• and / or that you have not "Expanded" your Settings -> Sound -> Volume panel style

then just feed the search engine of your choice with the buzzword: "in-call volume".
One found example is this one: http://www.droidviews.com/how-to-boostincrease-speaker-volume-on-samsung-galaxy-s3-note-2/
Here you could just decrease (instead of increase) the values inside the gain file if you like.

To check the volume-values you could use an app like "Sound Manager":
https://f-droid.org/repository/browse/?fdid=com.roozen.SoundManagerv2
[More Volumes] -> In-Call Volume, (and of course you could try to modify that volume here)...

I'm pretty sure the issue is that the lowest possible volume is still too high. No app can do anything about this, the audio module has to be changed to have a lower minimal value.

I could provide such a binary once our source code is back up.

My Self wrote:

If it is not just a settings-misunderstanding, for example:

• Settings -> Sound -> Volumes -> [X] Volume keys control ringtone volume (which should be disabled in your case)

It is enabled in my case.

• and / or that you have not "Expanded" your Settings -> Sound -> Volume panel style

It was expandable, which is similar so it does not seem the origin of the problem.

then just feed the search engine of your choice with the buzzword: "in-call volume".
One found example is this one: http://www.droidviews.com/how-to-boostincrease-speaker-volume-on-samsung-galaxy-s3-note-2/
Here you could just decrease (instead of increase) the values inside the gain file if you like.

Because they write "Do not try this method on Samsung Galaxy phone with AOSP, or any other custom ROM that already has volume boost hack." I think we already have this hack on Replicant. Am I wrong ?
I already noticed that we can increase the volume higher than secure level, by passing through the soundlevel warning.

To check the volume-values you could use an app like "Sound Manager":
https://f-droid.org/repository/browse/?fdid=com.roozen.SoundManagerv2
[More Volumes] -> In-Call Volume, (and of course you could try to modify that volume here)...

Thank you, I will check.
However, it seems one of my contact had a phone which was the source of this soundlevel issue, and it actually does not reproduce since he changes his phone, so I can not check if these solutions solve my issue since it does not exist anymore.