High energy coefficients in 1d DCT for audio
When we calculate 1D DCT of a signal (vector) we will get the DCT coefficients of same length.

The first value in this is the DC coefficient and low frequency components are at the begnning, as the index increases the frequency also increases. And hence the last value in DCT coeffs is the highest frequency (ie. fs/2).

Zig zag scanning is done in 2D DCT to read the coefficients in the increasing order of frequency. But in 1D DCT there is no need of zig zag scan as it contains only one row (or 1 column), it is already in ascending order of frequency (starting from Dc value and ends at maximum frequency ie. fs/2).