Then Kashiraj, great warrior Shikhandi, Dhrishtadhyum, Virat, Satyaki, Drupad, Draupadi's sons, other kings and the powerful Abhimanyu blew their own conchs.
Conch or Shankh (in Hindi / Sanskrit) is known as the manifestation of the sound, Aum. Blowing of the conch is to align one self along that sound.
Conchs are blown at auspicious occasions when something nice is being attempted. So, why would someone blow the conch at the beginning of arguably the most terrible war in history?
In a way, it is also an admission that Aum - a sound which combined the Brahm (Creator), Vishnu (Maintainer), and the Shiv (Destroyer); encapsulates not just what is good, but also what is terrible and even what we consider to be ugly about this creation.
Shankham" comes from the two Sanskrit words "Shum" which means something good and the "Kham" means water. Shankham, therefore means "Holder of good water".
Story of the Conch: the demon Shankhaasura had defeated the (elevated being) devas. And then he proceeded to the bottom of the ocean.
The devas went to Lord Vishnu and appealed to Him for help. Lord Vishnu incarnated as Matsya Avataara - the "fish incarnation" and killed Shankhaasura. The Lord blew the conch-shaped bone of his ear and head after killing the demon. That created the Aum sound emanated, from which emerged the Vedas.
Now, this story may not be a "fact", but it is the "Truth".
When the "good" in us (the devas) is rendered ineffective against the "bad" in us, then we have to rise above both. The access to our higher self - that which is beyond the good, the bad and the ugly, ultimately destroys the negativity of bad and the ego of the good.
For, it must have been instructive for the devas to know that the sound of Aum ultimately was created out of the remains of a demon they wanted to kill. The sound of Creation, from which all Vedas emanated did not arise from the devas but from the remains of the demon.
When the ego of good is no more and good in us is ineffective in handling the necessities of living, and the bad has been vanquished; then from that comes the path to our liberation.