HyperCard generates sound when a stack uses play or beep, or makes a sound because you typed a wrong key. With system software version 6.0.6, 6.0.7, or 7.0, HyperCard 1.2.x will lock up when sound is produced for any reason. When this happens, type the Command-Period (Command-.) key sequence. If you were editing a script when the program locked up, the script window closes; if a script was executing, it is canceled.
To prevent HyperCard from causing a lock-up condition, avoid making a sound. Set your Speaker Volume to 0 from the control panel. You won't hear any sounds, but your stack will avoid this sound problem.
If you must play a sound, you can use Farallon's fplay external command (and associated resources) from Farallon's HyperSound¬ Toolkit. If you use HyperSound, substitute fplay for every occurrence of the play command or beep command.
If you don't have to use HyperCard 1.2.x with System 6.0.7 or 7.0, don't use it. If possible use HyperCard 2.0. HyperCard 1.2.x is officially not compatible with System 6.0.7 or 7.0. So you may find problems other than those described above.