I thought I sent a reply but I must have messed it up.

I probably won't solve your problem but you didn't mention anything about your rig settings. I don't own an IC-7300 but there is some commonality with the IC-705. You probably have something like this on your rig.....Set >Connectors>MOD Input which should be set for DATA MOD OFF = either USB or USB, Mic and DATA MOD = USB (I'm assuming you are using a USB cable)

You can set USB MOD Level to something like 50% to start. I've read in the past that you should set it down around 20% but since you are missing modulation...start high into a dummy load and then set it lower once you find the modulation.
You never mentioned if the wsjt-x program was decoding signals or whether your waterfall display was showing signals. If not make sure that in Connectors, the AF Output Level is set to at least 50%.

There are several more settings in the Connectors group but I don't know what they are for the IC-7300 but if you just google IC-7300 settings for WSJT-X, I'm certain you will find them.

In WSJT-X Audio settins, you need to select Microphone USB Audio CODEC for the input and Speaker USB Audio CODEC for Output.

You also need to adjust the Pwr slider on the right edge of WSJT-X main page. Start at the top and if it's too high(too much audio drive) lower it.

If you still don't have drive, open the Audio Midi App and check to see if there are two USB Audio CODECS listed. Select the one with one or two outputs and on the right side of the app window verify that they aren't muted. The Audio CODEC with inputs isn't controllable on my mac.

You'll need to track down those other Connector settings.

Good luck