Ok, I understand the problem with MP3. In this situation, I think the best solution is making the sound file format converter in Unreal ('s idea). Any type of file can be dragged and converted into wave automatically. In this way, developers won’t use other file types in their projects that do not have clear licenses. Of course, somebody can say why not to convert with using other software (for example, online converters),. It’s true, but I know many users will be happy to see such a feature in Unreal. I am not sure which types should be converted, but for sure MP3. I’m not a specialist in sound technologies, but it would be great to have something like that.
The first found website with sounds:
All the sound in MP3. Unity Developers just download such sounds and use in their projects and nobody think he/she does something wrong.
Here, we can see .wav and .MP3 sounds.
And what with http://www.youtube-mp3.org/. We can get any sound or music legally using this service. I saw a tutorial about UE4 when a man used this service to get sounds from YouTube movie. Then, he converted the MP3 file to .WAV using an online converter.
Of course, I didn’t mean all Unity assets. The proprietary formats shouldn’t be touched. We can make better materials and other things in Unreal. However, non-proprietary assets should be imported to UE4 easily. Almost all is imported without problems. The problem with importing the characters (avatars with bones) that work in Unity well into UE4 refers to the skeleton incompatibility and I see the import frozen at 75%. But human models with Maya-style skeleton seem be imported without problems. But then they should be retargeted manually what is not good. I think Persona should be like Mecanim to detect most bones automatically. And there should be a feature to force the character to be in T-pose. Such a feature is in Mecanim.