AA batteries may vary from about 1.2 to 1.5+ volts, depending on the battery type. Four AA batteries would usually be in series, giving not more than about 6 volts at the battery pack. In that configuration, there is little danger that you'd get a "shock" you could feel directly from the batteries.

As noted, a 7.5 v battery is sufficient to produce an acid taste if you place your tongue on the terminals, but should be harmless except to the extremely sensitive. Those who get an objectionable effect should try to use their tongue more in excercises that will strengthen and toughen it, instead of in mindless prattle that does the tongue no real good. (What you've recently had in your mouth can have a significant effect on your sensation.)

As it's the earpiece that's actually connected to the body, it matters less what the batteries can do. The "player" can convert battery voltage to whatever is needed elsewhere in the circuit, although in cheap transistorised devices the battery voltage normally is the highest voltage needed anywhere in the circuit. Assuming 4 ohms or less for your earpiece speaker, application of sufficient voltage to destroy your hearing should not cause a "shock" hazard, so you won't die from listening to it until you walk in front of the taxi you didn't hear honking at you.

Telephones present a number of differing possibilities. A "landline" phone, with the handpiece connected to the phone base, can present 35, 50, or 70 volts in the "ring" and "busy signal" circuit. In dry conditions, it's possible to get a nasty shock from these voltages, and especially in wet places, in some bizarre circumstances, these voltages are sufficient to cause fatal effects.

RF phone handsets, where there is no cord, usually (in the US) have a battery that's not more than 12 volts, but may have an oscillator to step the voltage up to something higher for the required radio signals. Since any higher voltage is between points inside the handset, the voltages are floated with respect to the ouside world's "ground voltage" so most likely if you dunk the handset it will just short-circuit internally, without producing a shock to the user/occupant. If it's still going when you pick it up out of the water and stick it on your ear, you could get a shock sensation, but at the GHz frequencies common the current is unlikely (not impossible) for the currents to penetrate to where organic damage can occur.

Quoting rules about "this voltage/current will kill you and that voltage/current won't" can be very misleading, since the current path has a lot more to do with the organic damage than simply that there is a current. Shuffling your feet on the carpet and touching the radiator is generally quite safe, since the charge that's built up is on the surface of your body, and the discharge is across skin surfaces. Shuffling your feet on the carpet and touching someone else who's touching the radiator can - in very rare instances - be harmful to the person touched, since the same currents, applied at the single point of contact, could follow internal body paths.

Stepping off an airplane wing that's not grounded on landing has been lethal to many persons. Jumping off an airplane wing so that no simultaneous contact between airplane and ground happens at the same time has rarely been lethal, but can be very unpleasant and does have some very real potential for harm. (Voltages of 600 V and higher - to as much as 1500+ V - due to inflight static buildup are common.)

The bottom line on the cheap 4 x AA battery powered audio device is that it's unlikely to pose an electrical hazard in or around a pool, but that environment is very hard on devices of this kind. With anything electrical, the bizarre circumstance that disproves any rule should be anticipated.

John