When asked where in the world the most languages are spoken, many people would answer 'the island of New Guinea'; there are about 830 different languages spoken there. However, it turns out that there are around 800 languages spoken in New York City.
Many of the languages spoken are considered endangered, and three linguists have founded the Endangered Language Alliance (ELA) to document them -- they codify the grammars, pronunciation, and word-formation patterns and collect songs and legends in the language. Some of the languages they have been working on since founding the Alliance are Garifuna, a language spoken by descendants of African slaves, who ended up on St Vincent after a shipwreck, Mahongwe, spoken in Gabon, and Shughni, spoken in Tajikistan. Volunteers read words one at a time, and bring songs and other cultural material to the ELA. Sometimes things sound the same to the English-speaking linguists, but sophisticated software can often show up subtle sound differences.
The ELA has discovered some interesting things about the languages studied, for instance that in Mahongwe it is only tone that differentiates the affirmative verb form from the negative (so, for instance, the word(s) for 'I like' and 'I don't like' are the same, but they are spoken in a different tone, which is difficult for outsiders to even hear).
For more about this subject see this Economist article.