Debian installation: "The file format sln48 is not supported on your system"

Installed FreePBX on Debian using - wiki.freepbx.org/display/FOP/Installing+FreePBX+13+on+Debian+8.1
I’m not using the Distro.

After installation the FreePBX Dashboard instantly warns “The file format sln48 is not supported on your system”.

That’s probably the reason why I can’t play or upload sounds for “Music on Hold”.

No such command ‘file convert /var/lib/asterisk/moh/macroform-cold_day.alaw /var/spool/asterisk/tmp/temp.1482406486674.wav’ (type ‘core show help file convert’ for other possible commands)
File:/var/www/html/admin/libraries/media/Media/Driver/Drivers/AsteriskShell.php:157

The module res_convert which should provide asterisk’s ability to execute “file convert” is loaded inside asterisk and a installed sox package on Debian works properly.

How do I install the sln48 support? Is it specific for asterisk - a missing module to load or a missing package need to be installed on Debian?

There are multiple posts here with similar problems ending after suggesting to issue “asterisk-version-switch” - then the user says that he isn’t using the distro and everything is silent after that.

I already recompiled asterisk. Every single codec is pre-selected and res_convert is loaded after the installation.
It’s still not possible to play the preinstalled default files in the Music on Hold menu or upload new files there.

Edit:
managed to get preinstalled sound files to play:
go to your CLI and issue “asterisk -rvvv” - check if module “format_sln.so” is running. (module show like format_)

FreePBX Web GUI -> Admin -> Asterisk Modules - shouldn’t have recognized that format_sln is running
manually type in format_sln.so - Click on add - don’t wonder, no changes in FreePBX list of modules is made at all.

Now you should be able to play at least the preinstalled files listed in “Music on Hold”.

It’s still not possible to upload files or play files moved to the directory via CLI.

Unable to find an intermediary converter for /var/lib/asterisk/moh/$FILENAME.ulaw
File:/var/www/html/admin/libraries/media/Media/Media.php:297

[… - switching to asterisk CLI]

$HOSTNAME*CLI> module show like res_convert
Module Description Use Count Status Support Level
res_convert.so File format conversion CLI command 0 Running core
1 modules loaded

so, modules that should provide converting functions are enabled and loaded…