Obviously it's a little late to ask, but I only just noticed the whole Sounds section in the requirements. For future reference, what are the exact restrictions with sound? I'm not familiar with the limitations of Nokia sounds, and I'm a little confused about the one sound at a time rule, since hypothetically I could just write some code to make multiple sounds become one before playing.
To be safe I recommend using the sound pack provided on the resource page. Otherwise, people usually use a simple square wave synthesizer. Here is a good reference for what it should sound like / be capable of. As long as you try your best to replicate that kind of sound there shouldn't be a problem.
The Nokia 3310 could only play one sound / note at a time. So it wasn't able to produce chords, layered sounds, or have designated channels for sound effects and music to play simultaneously. The most it could do that you might hear in the video is very fast alternating between notes. The rules are simply reflecting that capability. Was that helpful or do you have more specific questions? I would be more than happy to try and answer them if you do.