External command does not execute completely - Java

So, I am building a program that converts .flv files to other formats.
For that I'm using ffmpeg which does its job perfectly when executing it via command line. For example:

ffmpeg -i C:\test.flv -acodec libmp3lame -y C:\test.mp3
 

This example works like a charm - there isn't a single problem when executing the command.

BUT when I try to execute the same command from within a Java class a problem occurs. I do this in a try-catch block:

System.out.println("Start");
 Process p = Runtime.getRuntime().exec("cmd /c ffmpeg -i C:\test.flv -acodec libmp3lame -y C:\test.mp3");
 System.out.println("End");
 

The console prints "Start". It starts converting and it doesn't finish.

Can somebody help me?

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