Hi chrisehrm,
I'm sorry to hear you're having audio issues on your EOS C300 Mark III. Going by the trouble shooting you listed it indicates there is an issue with the camera itself. Especially if the hiss has been at a consistent level in all of your recordings. Sometimes if a mic or cable isn't shielded it can cause hisses and static in the audio, but that is unlikely in this case since it is happening with multiple mics and multiple cables.
There are a couple of other things to check. One is to try loading different memory cards in the camera. Sometimes if the memory card is malfunctioning it can cause file issues like audio issues or artifacts in the image.
Another thing would be to try adjusting the Trimming and the Attenuator for the channel you're using. Those can help cut out lower level audio and in some cases can help with things like hissing.
If the issue continues after checking those things we would recommend sending the camera in for repair.
The Canon U.S.A., Inc. online repair portal allows you to set up service in a few simple steps. The repair portal gives you the ability to set up service for multiple pieces of equipment at once, and to receive a repair estimate in the majority of cases, depending on your model and issue. The portal also allows you to upload images or videos that will assist us in repairing your product. You can also select how you wish to be notified of the progress of your repair. The portal's service history section allows you to view all services that have been performed on any of your camera or video products since January 7, 2020.
Please follow the link below to access our online repair portal to arrange for service:
https://mycanon.usa.canon.com/signin
Instructions for Accessing and Using the Online Repair Portal
You will need to access or create your My Canon Account.
If you already have a My Canon Account and your product is already registered, simply click on the “Get Repair Services” link on the left menu and follow the onscreen instructions to arrange for service for your product.
If you have a My Canon Account and your product is not registered, you must register your product first. Click on the “Get Repair Services” link on the left menu, then click on the “Register Product” link on the right. After your product is registered, click on the “Get Repair Service” link on the left menu and follow the onscreen instructions to arrange for service for your product.
If you do not have a My Canon Account, you will need to create one. After you create your account, click on the “Add Product” link to register your first product. After your product is registered, click on the “Get Repair Service” link on the left menu and follow the onscreen instructions to arrange for service for your product.
Once you have completed the repair request, you will have an opportunity to print a copy of the request for your records. This request form will also contain shipping instructions and an address label that you will attach to the outside of your shipping box when mailing your equipment for repair.
