i think you need to give some examples. because sounds could be change by the flow. what strange ( different ) are different depends on what languages are your native. R and L, i understand what you mean. because most of us cannot pronounce R, L. and since it is not english, i have to say that comparing the sounds R and L are the first problem. you need to learn Hiragana and pronounce them correctly, we don`t have english like L and R, sounds.

For the "L" and "R" problem, this is just how they spell it. All "l" sound you hear it's basically "r". If you said love, they can't said love, but will be rove ^^ but this is just their habit ^^ And for the "g" this is just how they spell it.. sample "nagai" you will hear "na-nga-i"

Well... as you know, japanese usually don't pronounce the L sound. I mean, in fact, it doesn't exist in Hiragana. However... as japan is being very influenced by the american and western culture in general they often prounounce the R as L [in songs] (generally)... I don't know why.. maybe they think it's softier or prettier. As for the G and N/M, my japanese teacher (who is, in fact, japanese) said that it's more polite to speak the G sound softly, making it similar to an N sound (but not that, exactly). She also said many people can't do that sound anymore, or just don't (it's very "traditional"). That's it!