Grammar fix

tail off

gradually decrease (amount, strength, activity, or voice)

to become smaller, weaker, or less active in a slow and steady way, often until it stops completely

phrasal verbC1occasional

Hear it in context

Notice where the phrase sounds natural.

Attendance at the evening lectures usually tails off towards the end of the semester.

At first, the number of applicants was high, but interest tailed off as the deadline approached.

One small grammar move makes it sound right.

Not: His voice tailed ~~off down.~~

Say: His voice tailed off.

'Tail off' already means fade away — don't add 'down'.

Pattern
tail off (intransitive); tail off + (to/into + noun/number); tail off + (until/until + clause)

Tap the one a native speaker would say.

Words that usually travel with it.

tail off + decline