Reaper issues

Here are a few thoughts... and apologies if I have misunderstood anything.

Reaper may be recording to a better sampling frequency/bit-rate than that to which you are rendering. Check your render settings. Not likely to make that much of a difference, unless you're rendering to a lossy format.
Are you running effects that aren't included in the render...? Possible, but not  that likely unless you're using complex routing.
Are you listening on the same speakers...?
Are you playing back through the same soundcard...?
Are you sending your recorded sound through the monitors twice when listening before rendering...? This is easy to do with monitoring using your mixed and will obviously affect the sound.

A good test is to drop the rendered file back into a new Reaper file to listen to it exactly as you made the original recording, then listen for a difference.

I'm not bad with Reaper, and recording in general, and these are common issues, but there are likely others who will spot the problem immediately.

It sounds like you are mixing two versions of the file to your speakers. One directly from the computer and one from the mixer - and the phase difference is causing the sound to lose the "dynamics". Try playing the file from your wav player but straight into the speakers from the computer. The fact that it plays well in Reaper bears this out, as Reaper is ensuring the routing is correct.

What about playing the rendered file from Reaper directly into the speakers...?

I'm trying to isolate the effect, so we know if it's a file sounding great artifically, or if it's sounding bad owing to cancellation effects. Having exactly the same signal path for the pre-rendered and post-rendered sound would also highlight if there is a difference because of the rendering process itself. With that in mind, it's worth checking your render settings - and maybe even try another known file just dropped into Reaper to hear if there is a difference when it's rendered.

Hopefully others find this useful/interesting... if there's an issue, it can go private. Sadly, issues like this are usually hard to find and solved by clicking one button or clicking one box. Also, if I'm going in the wrong direction, I'm open to being corrected by those with more studiocraft.

OK, the difference that you're hearing can be either an artificial flattering of a signal path when monitoring for a mix/master (which is usual with a monitoring path that is boosting frequencies) or phase cancellation which is effectively removing frequencies that you heard when making your mix/master (which happens when there are signals that make up the sound are effectively working against each other).

Right, now I'm going to make a wild guess here, your problem very much sounds like phase cancellation, which robs the sound of its energy. "Dull and lifeless" is a common way to describe a sound that is suffering from phase cancellation. Next, you seem able to produce a good sound, render it and then listen to it, all via Reaper without any problems - so, I'll take another wild guess that Reaper is handling all of the routing correctly for you.

This then leads to the other application that you're using to play the file, which appears to be making the sound "dull and lifeless" - hence, another wild guess here, this application is not being routed correctly. OK, final wild guess, this application is being routed to your speakers twice, but the two versions are slightly out of phase (probably one is delayed slightly by taking a different signal path).

It may be that you're already at this point in your thinking, so then we need a solution   

I've had a quick read of the manual for your mixer (Behringer Xenyx X1204USB) and I imagine that you're using the USB connecion to route audio into your computer (i.e. Reaper), which copies the Main Outputs, and also again to route it back from your computer to your monitors (which should be connected to the Control Room Outs on the Mixer). Coreect me if I've got anything wrong there.

Now, as you don't get the phase cancellation when playing back with Reaper into your mixer, via USB, and into your monitors connected to the mixer (as you reported), then Reaper is making sure that there is no cancellation effect via the mixer. However, it would appear that the application you're using to play back the file that does exhibit the problem is not stopping the cancellation effect.

So, some things to try:

• Check that the application that does exhibit the problem is not somehow receiving audio and mixing it;
• Try switching the "2-TR/USB TO MAIN" button (p9 of the manual) to see if you are routing two versions (out of phase) of the sound to the speakers;
• Take a known commercial sound file and play that through both Reaper and the other application - do they sound the same...?;and
• Take a known commercial sound file and render it (to a different name), now compare that with the original in both Reaper and the other application.

Apologies that was long-winded (two cups of coffee needed here). Hopfully, by walking through it logically, you can at least isolate the problem and make it appear and (maybe even) disappear. A quick bit of Google-Fu did show me that people do have problems with the USB routing, and especially that switch I mentioned.

Here's a quick video from Behringer that describes the difference between the Main Outputs and the Control Room Out.