Just a little gfeedback ...
I've been here for about a week now and I have found it fun and I've learned a lot (especially when it comes to accented words). However, my main complaint is the programmed woman's accent. I live in Mexico and was taking lessons from a lady with beautiful enunciation and a melodic accent that was music to my ears. The recorded voice seems to be made up of sample sounds from which the phrases are generated. She seems to pronounce "yo" like "jo" or "go" with a soft G sound. Also her Bs and Vs are very hard to distinguish. Also translating "en el restaurant" as "in the restaurant" is marked wrong although "en" can be "in' or "at". (they translate "en la casa" as "in the house"). Overall it's been an enjoyable experience and I hope they can work out the kinks but I would also like to see them recognize that North American Spanish is very different from that in Spain (or Sounth America for that matter) and cater to those differences. For instance, they "una bola" is "a ball" but here in Mexico it is "una pelota". In spain the verb "coger" is used frequently but here it deemed offensive if you are not careful. Just a few thoughts.
I have found that since they put the link to playback sounds slower it helps me and I tend to listen to the sentences with the sounds you mentioned a few times before understaning. As for the differences in regions for certain words (ball) I think thats a very hard problem to solve. Do you have any advice for a North America learning spanish on the best way to learn about those types of differences?
I would venture that in the beginning, the more important thing is to learn the basic language, structure, pronunciation, etc and then at some point in the future the learner will naturally start to pick up the differences. I work with some people from the Dominican Republic and some people from Central America and the way they speak is very different, and it has taken some getting used to, but I don't think DuoLingo has the resources (yet?) to make different sites for "Spain" Spanish and a "Mexican" Spanish and an "Argentinian" Spanish. I would recommend working with what's here and you'll pick up differences as you go along.