相关疑难解决方法(0)

我尝试在 trusty/Ubuntu 14.04 中安装 ffmpeg 并收到以下消息：

$sudo apt-get install ffmpeg

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package ffmpeg is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'ffmpeg' has no installation candidate

为什么包不可用？

我的电脑上有很多音乐，但不幸的是它是 m4a，而 Steam 只需要 mp3。

是否有程序或终端命令可以转换整个文件夹？