Error

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
∗ installed conf-m4.1
∗ installed conf-pkg-config.1.1
∗ installed conf-libpcre.1
∗ installed dune.2.5.0
∗ installed camomile.1.0.2
∗ installed cry.0.6.5
∗ installed dtools.0.4.2
∗ installed dune-private-libs.2.5.0
∗ installed dune-configurator.2.5.0
∗ installed fdkaac.0.3.2
∗ installed ocamlfind.1.8.1
∗ installed base-bytes.base
∗ installed ladspa.0.1.5
∗ installed lame.0.3.3
∗ installed mad.0.4.5
∗ installed magic.0.7.3
∗ installed ogg.0.5.2
∗ installed flac.0.1.4
∗ installed mm.0.4.1
∗ installed samplerate.0.1.4
∗ installed sexplib0.v0.13.0
∗ installed base.v0.13.1
∗ installed pcre.7.4.1
∗ installed duppy.0.8.0
∗ installed taglib.0.3.6
∗ installed vorbis.0.7.1
[ERROR] The compilation of liquidsoap failed at
        "/usr/local/centovacast/.opam/opam-init/hooks/sandbox.sh build make".

#=== ERROR while compiling liquidsoap.1.3.7 ===================================#
# context     2.0.5 | linux/x86_64 | ocaml-base-compiler.4.07.1 | https://opam.ocaml.org#8fea66d0
# path        ~/.opam/4.07.1/.opam-switch/build/liquidsoap.1.3.7
# command     ~/.opam/opam-init/hooks/sandbox.sh build make
# exit-code   2
# env-file    ~/.opam/log/liquidsoap-5593-3a9654.env
# output-file ~/.opam/log/liquidsoap-5593-3a9654.out
### output ###
# Error: Signature mismatch:
# [...]
#              | `Samplerate of int
#              | `Sbr_mode of bool
#              | `Transmux of transmux ]
#        File "encoder/fdkaac_encoder.ml", line 54, characters 8-328:
#          Expected declaration
#        File "src/fdkaac.mli", line 101, characters 2-277: Actual declaration
# make[2]: *** [encoder/fdkaac_encoder_builtin.cmx] Error 2
# make[2]: se sale del directorio `/usr/local/centovacast/.opam/4.07.1/.opam-switch/build/liquidsoap.1.3.7/src'
# make[1]: *** [all-auto-ocaml-prog] Error 2
# make[1]: se sale del directorio `/usr/local/centovacast/.opam/4.07.1/.opam-switch/build/liquidsoap.1.3.7/src'
# make: *** [all-subdirs] Error 2



<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
┌─ The following actions failed
│ λ build liquidsoap 1.3.7
└─
┌─ The following changes have been performed
│ ∗ install base              v0.13.1
│ ∗ install base-bytes        base
│ ∗ install camomile          1.0.2
│ ∗ install conf-libpcre      1
│ ∗ install conf-m4           1
│ ∗ install conf-pkg-config   1.1
│ ∗ install cry               0.6.5
│ ∗ install dtools            0.4.2
│ ∗ install dune              2.5.0
│ ∗ install dune-configurator 2.5.0
│ ∗ install dune-private-libs 2.5.0
│ ∗ install duppy             0.8.0
│ ∗ install fdkaac            0.3.2
│ ∗ install flac              0.1.4
│ ∗ install ladspa            0.1.5
│ ∗ install lame              0.3.3
│ ∗ install mad               0.4.5
│ ∗ install magic             0.7.3
│ ∗ install mm                0.4.1
│ ∗ install ocamlfind         1.8.1
│ ∗ install ogg               0.5.2
│ ∗ install pcre              7.4.1
│ ∗ install samplerate        0.1.4
│ ∗ install sexplib0          v0.13.0
│ ∗ install taglib            0.3.6
│ ∗ install vorbis            0.7.1
└─

The former state can be restored with:
    opam switch import
"/usr/local/centovacast/.opam/4.07.1/.opam-switch/backup/state-20200414084118.export"
! Error: Failed to build Liquidsoap
! Error: Installation failed
Liquidsoap installation failed
Installer exited with error, aborting

Quick fix:

opam install fdkaac.0.3.1

before installing.

It seems that version 0.3.2 is not compatible with Liquidsoap 1.3.7...

I found I needed to use ffmpeg.0.2.1 and fdkaac.0.2.1 for a recent 1.3.7 install via Opam on Debian Stretch

fdkaac >0.2.1 was failing for some reason.

use
opam switch create 4.07.1

after

opam install taglib mad lame vorbis cry samplerate liquidsoap
opam depext taglib mad lame vorbis cry samplerate liquidsoap

opam depext opus
opam install opus