The 'unimodem half duplex audio device' is not a sound card. It is your voice modem's ability to function as a speaker or headset phone.
Open the box and remove and examine your sound card. It may be an ESS Audiodrive, Ensonique, SoundBlaster, Crystal, Acer/AOpen, Turtle Beach, MediaVision, or Analog Devices unit. Copy down the model number and chipset number, and use a search tool or drivers site to locate drivers for that model. When you have suitable drivers, use Control panel, Install new hardware to get the card back running in the PC.