Yes, absolutely. There could be several things happening in the connection to the mixer. The CP4 Stage has many stereo sounds, but more than that, many of the Effects are stereo. If the Mono Piano is louder it could indicate a problem, or it could just be that you are not in true stereo. When a stereo signal is forced to mono you can lose punch, crispness, and volume.

Try panning the Mono Piano hard Left, then hard right, if the signal moves side to side, then you have maintained the stereo path through to the sound system.

But if you only have one cable connected; or if you have two cables connected but they are summed to Mono; or if the two channels on the mixer are not panned hard Left and hard right... all of these things can degrade the sound.

If you are connected using just the Left/Mono Output, the Mono Piano might indeed sound louder. L/Mono Output combines L and R to a single phase coherent output.

A problem would be if the sound system is out-of-phase... this could be the cause. Certainly an Out-of-phase system might make the Mono sounds louder.

Keep in mind also that when you change voices on the CP4, the new voice brings along with it a cluster of other factory-default settings, including insertion effects and some but not all of the settings that are accessed via EDIT > PART > PLAY MODE.

So, for example, let's say you turn on the CP4 and you're in the default Performance 001, which by default uses the 01:CFX St voice, and you do nothing other than change voices to 04:CFX Mn. You might think that all you've done is change from a stereo version of the CFX sample to a mono version. In fact, though, the CFX Mn voice has brought with it a bunch of other factory-default settings. These include a different insertion Effect B (MSC > EQ501 for the CFX Mn voice, vs. PRE > Mic3B-3 for the CFX St voice), which has different EQ gain settings and output levels. A new voice also brings along its own Volume/Gain settings under EDIT > PART > PLAY MODE.

As far as I know, there's no way to "opt out" of this behavior ... i.e., you can't tell the CP4 "I want to change voices but I want all of my other settings left alone." So anytime you change voices, you have to assume that a number of other changes have occurred. Unfortunately, the only way to find them is to go menu diving.

The solution to this is to create custom Performances in which you save all your personal preferences for a particular voice. Then, when you want to switch voices, switch Performances, and you'll get the new voice and the other settings that you like with it. For example, you might have one Performance for your go-to CFX St voice, and another one for your go-to CFX Mn voice, and you might customize the Volume/Gain settings (among other things) for each of those.

OK thank you so much! I am not very good with this technical stuff. I just show up and play the thing and I know the overall sound of the keyboard is not at all clear and crisp like I expect a Yamaha to be. I will check it out next time I go over to the church but I am pretty sure it is set up only into mono.

Mono is a compromise. The full glory of the Yamaha sound is unlocked when you maintain the stereo imaging through to the audio system.

The CP4 Stage has a technology called SCM (Spectral Component Modeling)... and the sounds that use this technology do "bring along" some of their own components when you select the Voice. What makes the selection of the 1978 Rhodes is not just the Spectral analysis of the Rhodes Waveform, but the physical model of the 78 Pre-amp that gave the 1978 Rhodes its character.

When you select the RDII 78 Voice, you get both components, you can separate them, however, if you'd like to mix and match Rhides audio analysis with a different era Rhodes pre-amp model. But the 78 Rd data includes (brings along) the 78 preamp

On the acoustic piano, the "pre-amp" is basically an adjustable physical model of a boutique microphone preamp. Your adjustable component is really changing the characteristics of a mic pre-amp - check them out. Expremely musicsl EQs.