No time, but I did not want to skip a whole week from writing!
The gemara (Sotah 35) writes that Hashem was angry at David haMelech for saying, "Zmiros ha'yu li chukecha." How can you call Torah a "zemer," a song? If you stop thinking about Torah for a minute, it is gone -- Torah study demands constant diligence and concentration. It's not like "Row row row your boat." Hashem promised David that as a result of his words and the attitude they reflected, David would make an error in a pasuk that even children know. Sure enough, when it came time to transport the aron kodesh, David used a wagon, misinterpreting the pasuk in our parsha that says the aron must be transported on the shoulders of the Bnei Kehas.
Netziv points out that it's not that David forgot the pasuk. He knew that, "...Ki avodas ha'kodesh aleihem ba'kasef yisa'u." (7:9) Those words, however, can be interpreted in two ways. It might mean that when and if the Bnei Kehas carry the aron, they should do it on their shoulders, i.e. it's a din in the gavra. If others carry it, then other means of transport are acceptable. Or it might mean that when the aron is transported, the only way to do it is on the backs of Bnei Kehas, i.e. a din in the cheftza of the aron. David misinterpreted the pasuk as meaning the former instead of the latter.
Be that as it may, what in fact is the big deal in calling Torah a "zemer?" Isn't the mitzvah of writing a sefer Torah derived from the words, "Kisvu lachem es ha'shirag hazos?" G-d calls Torah a shirah -- so why can't it be called a zemer? And don't we say pesukei d'zimra every morning?
The word zemer shares the same root as the word zomer, pruning. If you want a plant or tree to grow, you have to first clear away all the tangle and bad growth that is strangling and holding back the good growth from developing. When we say pesukei d'zimra, what we are doing is clearing away all the negativity that is holding us back from connecting to Hashem. That's a wonderful and necessary goal, but it's not what Torah is all about, explains R' Yosef Englel in the Gilyonei haSha"s. Torah is not about just eliminating the pitfalls and restraints -- Torah is what brings the new growth and makes it happen. It is positive energy that moves a person forward. It is a shirah that uplifts, not just a zemer that cuts away the bad. (See also Pachad Yitzchak, Shavuos, #18)
R' Yosef Engel explains that when Yehoshua asked the malach who visited him on the eve of battle, as the gemara (Meg 3) tells us, whether he was there to chastise him for having missed the daily korban tamid or for missing learning Torah, what Yehoshua was driving at was whether what mattered more was having missed clearing away the negative energy that might bring harm, which is the goal of korbanos, or having missing the injection of positive good that comes from learning. The malacha answered, "Atah basi," alluding to the pasuk, "Atah kisvu lachem es ha'shirah hazos." It's the positive energy of Torah that was lacking.