Make xargs handle filenames that contain spaces

The xargs command takes white space characters (tabs, spaces, new lines) as delimiters.

You can narrow it down only for the new line characters ('\n') with -d option like this:

ls *.mp3 | xargs -d '\n' mplayer

It works only with GNU xargs.

For MacOS:

ls *.mp3 | tr \\n \\0 | xargs -0 mplayer

The more simplistic and practically useful approach (when don't need to process the filenames further):

mplayer *.mp3

The xargs utility reads space, tab, newline and end-of-file delimited strings from the standard input and executes utility with the strings as arguments.

You want to avoid using space as a delimiter. This can be done by changing the delimiter for xargs. According to the manual:

 -0      Change xargs to expect NUL (``\0'') characters as separators,         instead of spaces and newlines.  This is expected to be used in         concert with the -print0 function in find(1).

Such as:

 find . -name "*.mp3" -print0 | xargs -0 mplayer

To answer the question about playing the seventh mp3; it is simpler to run

 mplayer "$(ls *.mp3 | sed -n 7p)"

Try

find . -name \*.mp3 -print0 | xargs -0 mplayer

instead of

ls | grep mp3