When importing audio files into Alice, they should be as small as possible, not exceeding 3.6MB, this value may vary.
There are audio files with music and sounds provided as part of Alice, you can access them by clicking in the menu on “Project” and then select “Resource Manager”. The above panel will pop up, click on “Import Audio” and then browse and select the content you like to have. All these files do not exceed 1.6MB.
You can also import an own music file to Alice. Find a audio file in the internet, make sure that it is legal for you to copy it. If the file is in mp3 format, you can directly import it into Alice. You just follow the instructions above, locate your audio file as explained before and import it. Try to play the file with the built in audio player, If it is not playing, it most likely exceeds the size limit. Then you need to minimize the size of your audio file with programs as Garageband (Mac) or Audacity (Windows). The best way is normally to reduce the audio compression to a bitrate of 64kBit or 128kBit, both values give you still a sufficient audio quality. You may also shorten the playtime of your song. You can find instructions on how to do change the bitrate with the search term “compress audio audicity” or “compress audio Garageband”.
If you are curious to learn more about audio quality and compression algorithms, have a look here.