First test the sound in diagnostics (F12 immediately at boot) and if you do get sound then it may be a corrupt driver or Windows file. If you do need a sound card then this web page has a number of them and generally all require a PCIe x1 slot on the motherboard. As far as a video is concerned, try this one at YouTube, and there are a ton of similar videos there.

Does the 790 have a separate sound card or is the onboard sound being used?

Thanks All, but I believe I need a New Sound Card, I need exact instructions for replacing one, I was told mine is Integrated but still have to fix it. Any instructions would be greatly appreciated? Thanks 

If you really want to upgrade to a discrete sound card, get an old Del PCI card such as Creative Labs X-Fi Xtreme 7.1 Sound Blaster PCI Sound Card Model SB0770 or Creative Sound Blaster X-Fi Xtreme Gamer 7.1 channel PCI sound card SB0730

install it in white PCI slot assuming OP has 790 MT.

https://www.ebay.com/itm/373703179030?hash=item570271fb16:g:knwAAOSwiVphLSWw

https://www.ebay.com/itm/224619474486?epid=1906186524&hash=item344c5d2e36:g:KckAAOSwljhhS5y4

Thanks Redxp  btw i just got a private message from dell but i see no where to reply, How can i reply to a private message, anyone? Thanks

Reply option should show when you tap on the message.  If you're using a phone, in the browser menu switch to desktop site.  PM's are basically non-functional in Android otherwise.  Have the phone vertical and reply box should show below the message that's tapped on.  You may have to expand screen to be able to read anything.

Do you know which one exactly would fit the Dell OptiPlex 790 with the serial number # Thanks

I was able to find that you have the DT (desktop) model of the 790.  Therefore, the 2nd audio card redxps630 suggested would've been just fine, but it's sold (eBay).

It isn't just 1 or 2 cards that can fit a 790, but quite a few.  10's?  100's?  Sound cards are quite universal.  Depends on budget, number of ports wanted, sound quality, etc.

Onboard/integrated sound is irreplaceable.  How it works is when an audio card is inserted, onboard sound is automatically disabled.

Audio card goes in white slot of MB as redxps630 said.  Metal slot on back of PC that aligns with white slot is removed.

Since you have a DT, you need an audio card that's LP (low profile) that comes with a short bracket.

When video searching for installation, I went with "graphics card" instead of "sound card."  "...Sound card..." and "...audio card..." didn't pull up anything I liked in YouTube.  https://m.youtube.com/results?sp=mAEA&search_query=Optiplex+790+dt+graphics+card+install 

Optiplex 790 DT Owner's Manual  In it are instructions on installing/removing expansion cards.

Also:  Optiplex 790 Tech Guide 

quote>>>It isn't just 1 or 2 cards that can fit a 790, but quite a few.  10's?  100's?  Sound cards are quite universal.  Depends on budget, number of ports wanted, sound quality, etc.

SInce the ones on ebay are sold out, is there a list I can get of what sound cards work on this pc? That way i can search on Google? Thanks

There is No Sound when tested any where, If I unplug the speaker wire and plug it back in I get a quick humming noise, thats it.

Also, number of ports = surround sound options.

This card is from the web page JOcean suggested, looks like a nice one.  It also comes with a short bracket.  Creative Sound Blaster Audigy FX 5.1 Sound Card SB1570 https://www.amazon.ca/dp/B00EO6X4XG/ref=cm_sw_r_awdo_navT_g_6R8Z7H1YCT0VPA6APKTK

If you want to Google, just use "Optiplex 790 audio card" or Optiplex 790 dt audio card" for a search term.  You might find similar or the same cards come up.