I have heard people's cell phone go off when its in their backpack, but how does a soundwave travel through something that is completely zipped up?
I'll assume you want a short, easy to understand answer.
How easily sound waves travel through objects depends on a few different factors.
How much energy the sound wave has - how loud it is, but energy varies with frequency too.
How thick the object is - if the wavelength is shorter than the depth of the object, it will not get through(this is why you only hear the lower frequencies from next door's music).
How dense(massive/heavy) the object is(dampening) - with light materials, and materials with more air throughout, the sound wave will pass through easier, but with dense materials like lead, the sound needs a lot more energy to penetrate.
If the object is capable of absorbing the energy(absorption) - Some materials can efficiently absorb sound by taking the energy out of the wave, spreading it throughout the structure, and dissipating it as heat. e.g. a heavy, spongy material with a lot of fabric, like a mattress (This is also how acoustic foam works).
So basically; the sound wave generated by a ringing phone can travel through the thin, light material of a backpack quite easily. But if you wrap the phone in a jumper, then put it in the backpack, the sound wave generated by the phone, which is comprised mostly of high frequency(short wavelength) sound waves, will be (partially) absorbed by the relatively thick jumper, which dissipates the energy as heat(energy can't just disappear). So the ringing will be a lot quieter, if you can hear it at all.
I hope this answers you question.