I have a kaidaer mn-02 but the output for headphone the sound is in mono mode that would be in stereo mode with this output!

That would make sense. Places like BestBuy and Amazon have cables premade.

Stereo or mono headjack should not make a difference in the quality of the sound. The issue you have is with your head phones.

Bet that if you plug them into another apparatus they will sound lousy also. If they sound good, then the problem lies with the output stages of what ever you were plugging them into initially.

Sounds like it might be mono output. You can get a mono to stereo adapter, but of course you will hear the same in each headphone.

Some info from the manual:
The Dolby Digital and DTS listening modes can only be selected if your Blu-ray Disc/DVD player is connected to
the AV receiver with a digital audio connection (coaxial,
optical, or HDMI).
• The listening modes you can select depends on the format of the input signal. To check the format, see "Displaying Source Information"
• While a pair of headphones is connected, you can select the following listening modes: Mono, Direct, and Stereo.
• While Speakers A and B are on, you can select only the
Direct, Stereo, Mono, or T-D (Theater-Dimensional) listening mode.

Plus this Bit of info on the speakers A and B buttons.
While Speakers B is on, channels are reduced to 2.1 in the
main room.

If you can access the speaker configuration it should provide test tones to each speaker. If you can't the speaker A and B are not set right.
If you have pressed the stereo or mono button no sound will be heard except from the front two or the sub.

Did sound for a southern gospel group, using a yorkville powered mixer. Not familiar with your mixer, but, on the Yorkville I had an unused 1/4" line level output with the monitor mix on it. I bought a cigar-sized, 3-channel headphone amplifier / splitter from Radio Shack, called a Boosteroo. Has 1/8" stereo plug that I adapted to the mono output, with a 1/8" stereo to 1/4" mono adaptor. It receives the line level (monitor in our case) signal and supports up to qty 3 - 1/8" plug headphones. Battery operated. No volume control, so I added an inline dial (rheostat) volume control cable. Can't pull up on Radio Shack, now, but, found it available, elsewhere, online. Worked great for me!
Good Luck SdDude

The output of the casio may only be mono, you can buy splitters to create a 2x channel output. Is there any other outputs from the casio other than headphones, usually there are red/white RCA outputs (Phono's) which would be better for connecting to your soundcard.

Hope I was of use to you, good luck.

you might be using a mono output jack or input jack.....check the jack end...1 ring = mono / 2 rings = stereo
make sure your sending a stereo signal in, then check the L/R balance.

There is a possibility that the jack in the T.V. is wired for mono and the plug you are using is stereo. This means that the stereo plug would short circuit the jack and affect the output stage of the sound. These jacks were used on older T.V.s. Go to your local electronics store and see if they can sell you an adapter to convert the stereo plug to a mono one. Check your manual for the T.V. first to make sure that this is indeed the case. Hope this helps.

Hello Sound like you have a Mono jack output you can try some headset models that can be used Mono or Stereo.
http://www.tvps.com/Products/ECS-Deluxe-35-mm-Mono--Stereo-Over-Head-Headset---New__ECS-OH3-prd-5MSVC.aspx

http://www.tvps.com/Products/Lanier-MP-555-(425-3117)-35-mm-Dual-Speaker-Headset-with-Volume-Control---New-MP555__LA-MP-555.aspx

or if you don't like this models look for this Earbud Mono headset.
http://www.tvps.com/Products/ECS-Deluxe-DEB-35-mm-Transcription-Mono-Headset---New__ECS-DEB.aspx

1)probable incompatibility issue--for comparison did you try a set of headphones? 2)The RCA audio outputs on the tv can go to an amplifier that has speakers attached,ie surround sound. 3)Caution==never try to alter vol level at spkr!