If there are no speakers, paper cups can be used to amplify the sound. Although the DIY phone speaker won't be as good as a high-end sound system, they'll do the job just fine or at least enough to get the small party started. Making DIY phone speakers is easy as it is all about placing the phone inside something that is curved or tubular.
The DIY phone speaker amplifies sound because the sound waves move in one direction instead of scattering all around. Who knew!