I bet you, as a bot, won't understand this, but I do mean that I'm giving a SOLUTION. I have no problem now and want to share my experience with others. Also, if dell can listen to users, plz give Optiplex 7080 an audio console application. Such a way of configuring audio input & output is really hard to use.
Note that all pictures used is captured on my computer, which uses zh-CN locales. You can infer which thing to click from its position  1.open Dell Optimizer and click this: [IMAGE] and roll to the bottom and tick this box: [IMAGE] 2.plug some(proper)thing into the audio jack and this window will pop up. choose "headset" (1) and click "optimize" (2) tick "don't ask me again" if you don't want this window to show next time when you plug something into the audio jack [IMAGE] after this, the computer will use both the internal speaker and headset that was plugged into front panel and headset and internal speaker are shown as separated devices it's easy to make the internal speaker silent (disable or mute it)
A Solution to Optiplex 7080 audio problems