In my expert opinion, the chance of both left and right speaker cables not being connected is rather slim. However, if your laptop is not covered anymore, and you are interested in delving a bit deeper into your laptop, you can following the following guide to take your laptop apart to ensure that your Right/Left speaker cables are connected properly, and to ensure your speakers or cables are not physically damaged: If your laptop is still covered under warranty, I suggest taking it to the Apple Store to have it looked at and tested. This sounds like there is a hardware issue with either your speaker, speaker connector or Logic Board - If you are not getting any sound, especially after a PRAM/NVRAM reset, and not seeing any sound devices, then your speaker is either not connected to the board, or damaged.
