I cant say I have come across codecs problems with Music files but I can tell you thats quite common with video files. Usually simply installing the codecs required by your player fixes this for the videos.
The problem with them turning red is normal. You will find that some downloads or uploads will turn red. This comes down to making sure your set up properly and the user who is on the other end of the transfer is set up properly also. obviously there is not a lot that can be done about the other users.
For your set up I would ask you this. ..are you talking about uploads or downloads?
Do any uploads or downloads ever get through?
If the answer to the last question is no then you may need to forward ports in your belkin router to allow users to access your machine and allow it to act as a virtual server.
https://www.winmxworld.com/tutorials/routers_info.htmlIf some downloads and uploads are getting through its more then likely your set up is not to blame.
To improve download results have a look at this section on searching and avoiding fakes.
https://www.winmxworld.com/tutorials/filtering_fakes.htmlIf some uploads get through but most time out then you may also be dealing with an ISP that is filtering or blocking your P2P use.. provide me a bit more info and maybe I can help you more specifically on the time out issues..