sndfile-mix-to-mono: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/sndfile-mix-to-mono - from package: sndfile-tools

You may install the required package with command apt-get install {package}
Error in other languages:
sndfile-mix-to-mono: bevel nie gevind nie
sndfile-mix-to-mono: comandă negăsită
sndfile-mix-to-mono: comando não encontrado
sndfile-mix-to-mono: commande introuvable
sndfile-mix-to-mono: command not found
sndfile-mix-to-mono: käsku ei ole
sndfile-mix-to-mono: không tìm thấy lệnh
sndfile-mix-to-mono: komanda nerasta
sndfile-mix-to-mono: Kommando nicht gefunden.
sndfile-mix-to-mono: kommandot finns inte
sndfile-mix-to-mono: komut yok
sndfile-mix-to-mono: nie znaleziono polecenia
sndfile-mix-to-mono: níor aimsíodh an t-ordú
sndfile-mix-to-mono: no se encontró la orden
sndfile-mix-to-mono: no s'ha trobat l'ordre
sndfile-mix-to-mono: opdracht niet gevonden
sndfile-mix-to-mono: parancs nem található
sndfile-mix-to-mono: perintah tidak ditemukan
sndfile-mix-to-mono: príkaz nenájdený
sndfile-mix-to-mono: příkaz nenalezen
sndfile-mix-to-mono: команда не найдена
sndfile-mix-to-mono: командата не е открита
sndfile-mix-to-mono: 命令找不到
sndfile-mix-to-mono: コマンドが見つかりません