Problems related to synthesizer and Cakewalk

Hi and welcome to the MC4 forum! I'm not sure I understand all of your situation, but I use an Alesis QS7 synth and what you're describing is I think, the same kind of setup all of us with synth pianos go through.

My first thought is, that you may have both of your tracks set to the same channel, as that's what would cause them both to appear to be on the same sound. Remember, for each MIDI track you're trying to record to, you need to set each track to a separate channel, and then select your different patch - and on your keyboard, you set your channel to match the track you're recording (i.e., OUT 01, OUT 02, etc). I hope this helps; if not, please give us more details and we'll try to help further!

It seems like you're talking specifically about some sort of mismatch of sound patches that occurs during playback mode, but you're actually able to record OK? Do I understand the issue correctly?
If this is the case, are you sure you are using the correct Instrument Definition file for your X50? In your selection of the Instrument for your keyboard (I don't have MC4 open on this computer, but it's in the pull-down for Options / MIDI Instruments), it has to correctly identify your particular model synth so that the correct set of patches are fed into the software. If you're hearing them play correctly as you record, but not during playback, there's a problem with either the selection of patch being saved in each track, or I'd think it was a setting related to your Output of each Track into a Master Bus of the tracks you've already recorded. And you are sure that each of your other tracks of MIDI, are set to unique channels? Are you having this problem only with other MIDI tracks? Are any audio tracks playing with the correct soudns that you expect?

There's one part of what you said that confuses me: you said "whenever I use the desired sound chosen from the hard synthesizer to record a track, during the playback, the sound of the track varies along with the instruments I chose from the keyboard during the playback." Do you mean that when you play back a single track, it switches to various different patch sounds? I'd like to know about this before hazarding any further advice

Have you tried using the X50 editing software as a VST in MC? I don't know how that would work, though.
The Music Creator line of software does not support the importing of instrument definitions, so if it's not on the instruments list already, you can't use the ID for your keyboard. You'd have to upgrade to one of the SONAR Home Studio programs to import IDs. That's presupposing that an ID for the Korg X50 exists somewhere on the web for downloading.
You can only use the General MIDI patches of your keyboard, or a software synthesizer such as TTS-1 provided with MC 4 and later Cakewalk programs. To use the exact sounds of your synthesizer, you need to record and edit a MIDI track to your satisfaction, use no patch, set the keyboard to the sound you want, connect audio cords from the keyboard to your sound card and record the keyboard's audio output on an audio track in MC. Make sure you solo the track you want recorded as audio so all the MIDI in the song doesn't record on the same track.