Hi @kobosh ,
According to your description and error message, this issue is related with you are using the static path instead of using relative path. This is the reason why you get this error.
I suggest you could try to modify the path like below:
For example:
If your mp3 file is inside the wwwroot/sounds folder, then you could directly use below codes to get the mp3 file.
~ means the application's root folder, in blazor server app it means wwwroot folder.
<audio id="player2">
<source id="playerSource2" src="~/Sounds/aaa.mp3" />
</audio>