I’m frustrated

Audio sounds muffled when using AUX port on Alesis Forge Kit

We just got the Alesis Forge kit and are having trouble hooking up our Iphone or computer to the AUX port to play audio through the module. We tested the cable and it works with clarity when we hook it up to the computer and a speaker. Then we tried from the iphone to a speaker. Finally, from the iphone into the module, it sounds muffled... like its underwater. We want to be able to have headphones plugged into the phones jack and then plug in the iphone or computer into the AUX port. Please help.
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