8 being the highest water ingress protection rating
Not any longer. Things moved on a while ago.
The IP rating of a device is made up of two digits; the first representing protection against solid particle ingress, and the second against liquid ingress:
First Digit – solid objects, dust etc
Number = Protection level
0 = No protection against contact and ingress of objects.
1 = Any large surface of the body, such as the back of a hand, but no protection against deliberate contact with a body part.
2 = Fingers or similar objects.
3 = Tools, thick wires, etc.
4 = Most wires, screws, etc.
5 = Ingress of dust is not entirely prevented, but it must not enter in sufficient quantity to interfere with the satisfactory operation of the equipment; complete protection against contact (dust proof).
6 = No ingress of dust; complete protection against contact (dust tight).
Second digit – water, etc
Number = Protection level
0 = No protection.
1 = Dripping water (vertically falling drops) shall have no harmful effect.
2 = Vertically dripping water shall have no harmful effect when the enclosure is tilted at an angle up to 15° from its normal position.
3 = Water falling as a spray at any angle up to 60° from the vertical shall have no harmful effect.
4 = Water splashing against the enclosure from any direction shall have no harmful effect.
5 = Water projected by a nozzle (6.3mm) against enclosure from any direction shall have no harmful effects.
6 = Water projected in powerful jets (12.5mm nozzle) against the enclosure from any direction shall have no harmful effects.
6K = Water projected in powerful jets (6.3mm nozzle) against the enclosure from any direction, under elevated pressure, shall have no harmful effects.
7 = Ingress of water in harmful quantity shall not be possible when the enclosure is immersed in water under defined conditions of pressure and time (up to 1m of submersion).
8 = The equipment is suitable for continuous immersion in water under conditions which shall be specified by the manufacturer. However, with certain types of equipment, it can mean that water can enter but only in such a manner that it produces no harmful effects.
9K = Protected against close-range high pressure, high temperature spray downs.
For example, a product marked IP53 would be
Dust = 5 = Ingress of dust is not entirely prevented, but it must not enter in sufficient quantity to interfere with the satisfactory operation of the equipment; complete protection against contact (dust proof).
Water = 3 = Water falling as a spray at any angle up to 60° from the vertical shall have no harmful effect.
In this particular case, bearing in mind that a pair of Bluetooth headphones is very likely to contain a lithium battery, as Jim B suggested, I'd want to see an IP rating of at least x8. I'd avoid anything not declaring an IP rating.
Pete