I have a Vostro 3910 , what type of audio card can I add?

I have always been partial to the SoundBlaster cards. If you do not want to spend a lot of money then the Audigy series or even the Soundblaster Z should be fine. They all require a PCIe x 1 slot. You should have a x 1 slot available but pop the cover and have a look at the MB. There were 2 available MBs in that series. One had a PCIe x 16 and a PCIe x 1 slot. The other had the PCIe x 16, 2 x PCIe x1 slots, and a PCIe 32 slot. The picture of the MB can be seen here in the service manual.