Learn Finnish: Bändi kuulostaa äänekkäältä.

This is because of the verb kuulostaa (to sound). In Finnish, verbs of perception (like sounding, looking, or tasting like something) require the following descriptive adjective to be in the ablative case. That means adding the -lta or -ltä ending to the adjective.

Why is the ending -ltä instead of -lta on äänekkäältä?

This is due to vowel harmony. The stem of the word (äänekkää-) contains the front vowel ä and the neutral vowel e. Because there are front vowels present, the ending must also use the front vowel version, which is -ltä.

What is the base dictionary form of äänekkäältä, and why does the word change so much?

The base form is äänekäs (loud). Words ending in -käs go through a significant stem change before you can add endings. The final -s drops, the vowel before it doubles, and the single k undergoes reverse consonant gradation to become a double kk. So, äänekäs becomes the stem äänekkää-, to which we add the -ltä ending.

Is bändi just the English word "band" with an -i added?