BASS ACKWARDS

Choose any number of willing creatures within range. For the
duration, as long as the targets remain within the spell’s area, their
words sound scrambled and nonsensical to any listener who isn’t
under the effect of the spell. The chosen creatures can understand
each other as normal.
A creature attempting to eavesdrop on the targets of this spell
with a divination spell or effect, such as scrying or clairvoyance, must
make a Wisdom saving throw, taking 3d8 psychic damage on a
failure and half as much on a success.

At Higher Levels. When you cast this spell using a spell slot of
5th level or higher, the duration is concentration, up to 8 hours.
Using a spell slot of 7th level or higher grants a duration of 24 hours
that doesn’t require concentration.