Learn French: Tu chantes fort, mais la fréquence de l'écho est mauvaise. - You sing loudly, but the frequency of the echo is bad.

Certain adjectives act as invariable adverbs with specific verbs. For sounds, chanter fort and parler fort are the standard expressions for singing or speaking loudly. The adverb fortement does exist, but it means 'strongly' or 'heavily' (as in fortement conseillé), not 'loudly'.

If the subject were feminine, for example Elle chante, would fort become forte?

No, it would stay exactly the same. When an adjective like fort is used as an adverb to modify a verb, it is invariable. It never agrees in gender or number with the subject.

Since écho is masculine (un écho), why is the adjective mauvaise in the feminine form?

Look closely at the sentence structure: mauvaise is modifying la fréquence (which is feminine), not l'écho. It is the frequency that is bad, not the echo itself.

Could we use mal instead of mauvaise at the end of the sentence?