Hi! :) english is my second language, and I've never even been to an english speaking country, but I've just heard the language for so long (through tv and music) that I've been getting pretty used to it. anyways, I'll try to answer your question:
the american accent sounds like you have some kind of gum stuck on your palate, making you unable to open your mouths properly when you speak :) it sounds sort of cool though. and sorta nasal. you always hear when there's american coming from far away, I don't know if it's the nasality of your voices or simply because you're kind of loud :P
the british accent sounds more posh and more melodic, and also very authoritative (I guess that's why there's always a brit narrating nature shows and stuff like that) . I like british accents, they sounds really good. australians sound very happy, since they always go up in tone at the end of each sentence. I think australian sounds very happy and cute :)
I'll ask my mum what she thinks english sounds like, her english is not that good so maybe she'll have a better answer:
my mum thought british english almost sounded a bit old fashioned, what she gave as example was "a cup of tea", so I guess she associates it with tea as well, lol. american english she said sounded very nasal, and she also said she thought it sounded kind of ugly. when she mimiced it she did like "whee whee whee" and then added that the american accent sounded kind of whining. she said she thought it was quite a big difference between american english and british english. she didn't know what australian english sounded like, so she couldn't really give her opinion on that.
anyways, that's all I got, hope this helped some at least :)
me and my mum