Quicktime only play one file at a time

Is there a terminal command to only have one instance of quicktime at a time?

Not a big Terminal user, but as far as I know, the answer would be "No!" Nor does the QT Player (QT X) have any preferences that would help here.

I would like to use QuickTime player to play mp3s. when I play a file and then open another it opens another instance of Quicktime and both files play at the same time.

With the QT Player (QT X) the specified media player, you have three basic options:

1. Create a reference playlist or pseudo-playlist,
2. Create a standalone "album" file, or
3. Create a an Automator script that will play any group of folders and/or individual files dropped to it.

For instance, if you have QT 7 install on your system and is is keyed for "Pro" use, then you can use it to create a "reference" (pseudo-playlist) or "standalone" (album" file that will play the content you add in the order you add it to the player. The "reference" file is very small since it only tells the QT Player to play the source files located else where on your system but is not, generally speaking, "transportable" since files are easily "orphaned" by inexperienced users. On the other hand, the "standalone" contains all of the original data stored internally and can be move or shared across systems and media devices having compatible media players. Both types of files are fixed with respect to both the content and its playback order.

An automator "drop" script works like the playlist or pseudo-playlist file described above but has the advantage of allowing the user to grab and combination of files and/or folders and play them sequentially in the order (I believe) in the order they are selected. Its major disadvantage is that the user must select, drag, and drop the set of files and/or folders each time he or she wishes to play the content whereas playlists, "pseudo-playlists," and "albums" play the same files in the same order each time they are opened.

Most users prefer an alternative solution—that of using an alternative player. Most use iTunes since it also "manages" the media content. Music albums are, in and of themselves, playlists. However, users can also create smart or manual custom playlists as desired. These playlists are "reference" playlists by nature but can be exported, burned, streamed, or sync'd to mobile devices if desired by the user and thus, to my mind, offers the most available options to the average user.

Another alternative would be to use a third-party media player having "playlist" capabilities. One popular example would be the VLC media player. Folders, files, albums, etc. can be dropped to the app player window and automatically begin playing in the order dropped. Further, the playlist can be saved as an M3U playlist file that can be recalled by VLC (or iTunes) later if desired. Unfortunately these M3U files are not QT X or QT 7 compatible. (NOTE: The M3U file, like the QT 7 "reference" file mentioned above, merely contains pointers to data stored elsewhere on your system and any change to the relative navigation path can orphan the resource data.)