If you use Snack to play audio from tclsh instead of wish, then asynchronous play works only if the event loop has been started with, for example, the vwait command, see aserver.tcl for an example. Note that use of the after command will cause the event loop to block. The visualization part of Snack needs Tk. If you only need to play sound you can use the command package require sound to get only the audio functionality of Snack. Depending on your local configuration you may or may not be able to load Snack into Tcl, regardless of the fact that some parts of it requires Tk.
Synchronization is dependent on sound hardware and device drivers.
The behaviour of the pause command is a bit platform dependent, on some platforms playback stops only after a short lag. See the xs.tcl script for an example of this. Simply install Snack for Tcl v8.0 in the default location. Skip the installation of Tcl, as it is already contained in the CSLU toolkit distribution. Send an e-mail with your question, try to give a full explanation, including information such as Tcl version, Snack version, platform.