HDMI (High-Definition Multimedia Interface) is a digital audio/video interface that can support up to 8 channels of uncompressed audio, including 5.1 surround sound. If both your Sony DVD player and your 46-inch plasma TV support HDMI connections, and you use a high-speed HDMI cable, you should be able to get 5.1 audio through the HDMI connection.
Using a coaxial cable:
A coaxial cable is a type of analog audio/video cable that typically carries a single stereo audio signal (left and right channels). It does not have the capacity to carry multi-channel surround sound formats such as 5.1. Therefore, if you connect your Sony DVD player to your TV using a coaxial cable, you will only get stereo audio, not 5.1 surround sound.
To summarize, if you want to get 5.1 audio from your Sony DVD player to your 46-inch plasma TV, you should use an HDMI cable connection rather than a coaxial cable.