No audio output. muffled output from Phones jack

Yes. But it depends which connectors you have on your laptop. If you have a small (3.5mm) stereo jack input, then you need one cable with a small stereo 3.5mm jack on one side and a big stereo 6,3mm jack on the other side. (Take a look at the picture below).
Then you plug in side with a small stereo jack in your laptop "line in" input and other side of the cable with a big stereo jack to your Casio CTK-1100 Phones/Output plug on a keyboards rear side...

Be sure to adjust the volume on both the Casio (output volume) and laptop (input volume)...

Cheers!

Is your graphic eq assigned to main and is kit set correctly?

Generally mono outputs on this kind of mixer are usually balanced. This means all three poles of the connector are connected. The hot or plus signal (tip) will go to the left of your phones, the cold or minus (ring) will go to the right. This will be ok from a sound point of view But it may not have enough drive to make the phones loud enough.
If you only hear a signal in the left then the output is unbalanced.
A simple fix is a cheap headphone amp (less than 20 pounds) and a simple jack to two phono lead (wired as a split). There is a ton of old audio equipment about for very little money including some very small mixers, that have headphone outputs, that is what I have done a lot in the past.

I need more info to correctly troubleshoot this. However my best guess from what your telling me is that its either the Audio jack on the Korg or the headphones. If everything else is working properly replace the headphones and the Audio outputs. Post again with more details if that does not work

I think that rather than the mixer, that your front speakers may have a limiter built into the high frequncey amp. You may be asking them to do more that they are capable of.

You should NOT under any circumstances connect an audio input lead to a speaker output jack!
The RCA jacks you are trying to take output from ore "pre fader" which would be solid "line level" and not controlled by volume controls. Tap off the INPUT to the amp... you should be using balanced lines for the interconnects. RCA jacks are unbalanced lines. You can drive several inputs from a line level output of a mixer for example using a splitter cable.

Hi. You can use standard 1/4 inch jack to jack audio cables (ordinary guitar leads will work fine) to connect the MIX outputs to a keyboard combo amp. If the amp only has a mono input, just use one cable from the MIX output marked L to make the connection. Remember to keep the amp volume turned down until you have plugged in the cable(s) from the K2500S. All best.

This probably is not a DIY repair... The audio comes out of the CODEC and is buffered by Op Amp used as filters and then goes through the volume controls, After that it goes to the jack board with buffer OP amps. First thoughts might be a damaged volume control, however, the "B" outputs don't go through volume controls...

About the only thing I would think you could do is to CAREFULLY open the unit and look for a loose cable involved with sending the audio to the volume control panel and then the cable that takes the audio to the jack board...

More than that you would probably need to take it to a shop equiped to troubleshoot.

Just because tubes light up does NOT mean they are good or working... While heaters not lit are an indication of failure, teh reverse is not necessarily true.
Time to troubleshoot this... Plug a set of headphones into teh preamp out. See if audio there is clean... it will be low level using headphones and ONLY from one side as the output is mono and your headphones are stereo usually... If audio is clean here and not muffled, then problem is in power amp section else in the early stages of the unit.
Report back the results of this test...

The probable problem is poor connections at the input and/or audio output jacks.

Usually the power supply will fail either on or off which is sometimes cause by a bad input power jack.

In any case, the problem is likely internal and will require repair.