E250 volume fails and dial won't work

Given the troubleshooting steps you have already performed, the only other thing to try would be a different set of headphones. If that is not successful, checking and updating the firmware is a last ditch effort. Failing at that, you're looking at failed components and either official repair/replacement or to be comfortable with component-level work with the innards using a multimeter and soldering iron and hoping that you can find what failed.

What year is your Envoy ? Probably high speed relay .

If you indeed installed the positive battery cable last and the neg first you blew the computer possible, you will need a complete diagnosis.

I have the same two problems! "Zone 2" works, but the main amp will make a noise and then stop working. Sometimes when I change the listening modes around it will start working. Lately, though, "zone 1" has gone out altogether.

make sure you have headphones plugged in. Once your in the screen for music or radio turn the clear dial (that looks like a border but isn't) clockwise to turn the volume up.

turning on to change modes is your problem. If it's completely dead you may need to replace it or refurbish it. Or if the battery is completely dead plug it in all day until you can get it to turn on. ONCE ON then you can worry about getting it to connect. Try this:
There are two connection settings in USB: MTP and MSC. For some reason MTP is the default, but it doesn't connect properly on all computers. Select MSC and plug the player in again, the computer will recognize sansa e250 but not show the driver just yet. It is installing recognition software (just basic drivers - nothing heavy) and it doesnt tell you what its doing until its complete. WAIT FOR IT. Once it is complete you will see both the sansa e250 and the removable memory slot in your "my computer" now you're good to go.

Try the code below at your own risk. I will not be liable for any damage this may cause to your phone.

Type the following codes on standby menu

*#0206*8378#

after typing you will be presented to a Test Mode menu
Select H/W Test
Select Audio Settings
Select IIS Normal (option 10) this is the normal volume for the phone. Scroll down to select Rx vol. then change level 14 to a higher value. Maximum is 30,000, i set mine at 5000
You can do the same for IIS Normal2 this is the volume for Music Player

e250 mic not working

RESTORE UR COMPANY SETTING OR CHANGE ANTINA.

Volume + while connecting to computer will send Windows into recovery mode