Lulav

I have two questions about the mitzvah of the four species.

1) The species involved are weird. One would expect that they include species common to the land of Israel, like grapes or olives. But none of the "seven species" appears in the four species. In fact, the four species have very little importance in Tanach in any context except for this mitzvah.

2) Chazal say that the four species represent four types of people who must be united for the mitzvah to be performed. What is the source for this midrash?

To answer these questions, we must look at where each of the species grows.

The lulav is part of the palm tree, which grows in the desert.
The etrog is a citrus fruit, which grows in the coastal plain (like the Jaffa orange).
The arava grows in valleys ("arvei nachal").
The hadas grow in mountainous regions.

We see that each of the species grows in a different part of the land of Israel. A person living in Ein Gedi, for example, would have plenty of palm trees around, but no etrog trees. Meanwhile, a person living in Kfar Sava would have plenty of etrogs but no lulavs. When everyone gathered to Jerusalem for Sukkot, the Kfar Sava people would have to swap their etrogs for the Ein Gedi people's lulavs, so that each group would have both lulavs and etrogs. Similarly, they would have to find people from the mountains (Hevron?) and valleys (Afula?) in order to obtain hadasim and aravot.

It seems that the four species is a mitzvah that no individual could perform on their own. Only one of the four species would grow in their hometown, and the others would have to be obtained from people living in other parts of Israel. Thus, it was a mitzvah that had to be performed by the entire people, working together, coming together to Jerusalem.

After the exile, Jews no longer lived in each part of the land of Israel, and they would not all gather to Jerusalem on the holidays. Thus, it no longer made sense to talk about the mitzvah as uniting people from different geographic regions. Instead, Chazal said that each species represented a person of a differing type of piety - and people of all such types can be found in any place.

(Source)

Falling on your face

For an interesting explanation of Moshe and Aharon's falling on their faces, which happens a number of times in Sefer Bamidbar for unclear reasons, see here.
In short: People fall on their faces when they notice a Divine revelation, lest they see God and thus die. Moshe often falls on his face when no revelation has yet occurred. He does this as preparation for a revelation he hopes will then happen, because such a revelation would be an effective answer to the complainers or rebels he is confronting.

Kol Isha

"A woman's voice is nakedness" (Brachot 24a)

The justification of this statement is not obvious to everyone today. Sure, many people go to concerts in order to swoon over the performer of the opposite sex, but there good looks and suggestive dancing are at least as much an attraction as a good voice. And those performers are generally required to have relatively good voices. For the rest of us, with our often ugly voices, the attraction is clearly much weaker. Yet Chazal called that voice "nakedness", and prohibited listening to it in the same line in which they prohibited staring at a woman's leg!

To me, this is best understood by reference to Greek mythology. Measures of sexual attractiveness are influenced by one's culture, and the Greeks and Romans whom Chazal lived among found the female voice to be very seductive. I am thinking of the myth of Odysseus and the Sirens. There are, it is told, sea-dwelling creatures which sing in a woman's voice, whose song is so seductive that any man who hears them will have an irresistible urge to abandon ship and swim to them, only to be killed. Unlike the mermaids of other cultures, who often had beautiful bodies, the sirens were attractive only because of their voices. The rabbinic decree of kol isha responded to what, at the time, was apparently a very strong temptation, as hinted to by the myth of the sirens.