Continuous sound is difficult for three reasons: you must continuously generate the sound samples, then push those samples to the sound card driver, and then the sound card hardware needs to fetch them.
Assuming you can generate your sound samples faster than real-time and have enough memory to buffer say 10s worth of sound, then even if the computer does something (like defragment your hard drive or scan for viruses), you should be okay. The more you buffer, the safer you are.
Transferring the data from the soundcard driver to the hardware can be problematic and is out of your control. USB and firewire devices have limited bandwidth. High sample rates and high channel counts can tax the system and result in dropped frames. You cannot guarantee that frames will not get dropped without specialized hardware. Under Microsoft Windows, ASIO cards can let you know if a frame gets dropped. If your computer is not doing anything special, your sample rate is 44.1 kHz, and you have 2 channel audio, then you will not drop very many frames. When your computer scans for viruses, you will drop more frames. You will also drop more frames if generating the sounds is time consuming.
You can more or less continuously stream a sound (up to about 2^32 samples) with the right interface. Neither sound nor audioplayer let you do this. You can use the winsound device in the Data Acquisition Toolbox. Even better is to use a port audio sound interface. I prefer the one with psychtoolbox, but pawavplay and playrecord also work.