Just multiply the number of minutes in a song by 60, and then add the seconds to that number. That's how many seconds there are in a song.

So a song that's 3:23 long, is 203 seconds long.

If you have a bunch of songs, do the same then add up the seconds. Divide that number by 60 to get your minutes. The remainder is the seconds. So if a bunch of songs add up to 3692 seconds. In minutes and seconds thats 61:32 long.

And no I don't know of any software, but it's not that hard. If your adding up media files (like mp3's, or wavs), just put them all in your playlist on a media file player (like winamp) and it should tell you.