mpg123-oss: command not found

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

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

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/mpg123-oss - from package: mpg123

You may install the required package with command apt-get install {package}

Error in other languages:
mpg123-oss: bevel nie gevind nie
mpg123-oss: comandă negăsită
mpg123-oss: comando não encontrado
mpg123-oss: commande introuvable
mpg123-oss: command not found
mpg123-oss: käsku ei ole
mpg123-oss: không tìm thấy lệnh
mpg123-oss: komanda nerasta
mpg123-oss: Kommando nicht gefunden.
mpg123-oss: kommandot finns inte
mpg123-oss: komut yok
mpg123-oss: nie znaleziono polecenia
mpg123-oss: níor aimsíodh an t-ordú
mpg123-oss: no se encontró la orden
mpg123-oss: no s'ha trobat l'ordre
mpg123-oss: opdracht niet gevonden
mpg123-oss: parancs nem található
mpg123-oss: perintah tidak ditemukan
mpg123-oss: príkaz nenájdený
mpg123-oss: příkaz nenalezen
mpg123-oss: команда не найдена
mpg123-oss: командата не е открита
mpg123-oss: 命令找不到
mpg123-oss: コマンドが見つかりません