Hearing yourself play while playing back other tracks?

Do you have the Input Monitor button on your track headers, it's a capitol I in a small orange (when activated) box, shaped just like the record button. If not go to the drop down "View" menu in the arrange page and select "Configure Track Headers". Add Input Monitoring. You should hear input all the time when this button is enabled and software monitoring is enabled.

What audio interface are you using?

There's something you should understand... you have two very different configurations to achieve the same goal.

You can enable Software Monitoring in Logic and monitor through Logic's audio engine or you can disable all of the Logic options (software monitoring, input monitor..etc) and monitor entirely through your interface using the built in DSP for effects. You will still be able to hear playback from Logic but the C400 had the ability to monitor it's Inputs, plus add DSP effects to your sound. This is a low latency solution.

Of course, if you are playing a Logic Instrument you must go through the audio engine and software monitoring.

No, it's not bad at all... what I described is a feature.... your interface has the ability to monitor it's inputs without going through Logic's audio engine... not only that it has built-in DSP (digital signal processing) which means you can add something like Reverb for a vocalist and yet record a dry signal into Logic to add other effects later.

Plus, this type of monitoring is very, very low latency, Lower than Logic can manage... the downside of this kind of monitoring is you can't use Logic effects as the monitoring system is the interface, not through Logic's audio engine. otoh.. your interface can function just fine as an interface that is monitored through Logic... turn of the direct monitoring system and DSP on the interface, enable Logic's "software monitoring" set the I/O buffer to 128 and you can now play Logic's software instruments and hear Logic's effects.

It's a good system, the direct monitoring would be used when you had live external input like a singer, sax, brass, the built in C400 reverb will be enough to make the musicians feel comfortable recording and latency will be near zero.

You have a manual for the c400, right. It has some decent features you might want to be familiar with.

should read:

... turn off the direct monitoring