The answer is:
Because the lines of this riddle represent:
definitions of words that can be made by removing 1 letter at a time, starting with DICTIONARY and ending with I. The numbers in each line indicate the word length.
10 is the password,
the weapon of choice;
DICTIONARY - The puzzle tells us that we require an English dictionary. The 10-letter word 'dictionary' is both our password and our weapon of choice for this puzzle, then...
9 tracks your progress,
INDICATOR - This part of a car displays to other drivers which way you are about to turn (i.e. tracks your progress).
and 8 the sung voice.
DIATONIC - A scale in music.
7 tracks your speech;
then 6 will accuse.
DICTION and INDICT.
5's bright and 4's hollow;
NITID (bright and glossy) and DINT (a dent or a hollow).
3's nearly sin if I lose.
2 is in 3 and
in this riddle thrice.
3 is either DIN or TIN. If 'if I lose' represents 'shifting a character down by 1' then TIN would become 'SIN' and fits best, but there's an argument for interpreting it in the context of a QWERTY computer keyboard, in which case moving 1 along from S gives you D, hence DIN...
Either way, 2 is IN - it's found within 3 and also in the riddle three times, twice by itself and once within 'sin'.
1 is the solver,
a char will suffice.
Remove the 'N' and I am the solver! 'A char will suffice' here, since the 'char' data type in various programming languages stores a single byte of memory, enough for a single character - in our case, the letter 'I'.