Can you review electrostatic potential energy?

When you bring two like electric charges together, they repel and you must do work on them to get them nearby. They store this work as electrostatic potential energy. If you let go of them, they'll fly apart and this stored work will be released as kinetic energy. The same sort of activity holds true if you pull two like charges apart.

When astronomers study sunspots they occasionally notice that there only seems to be one magnetic pole. But I thought that monopoles didn't exist that we know of. What's going on?

While a sunspot may have only one magnetic pole associated with it, there is sure to be an equal but opposite pole somewhere else in the sun. Probably it's located deep inside the sun or somewhere else on the sun's surface. Like one end of a long bar magnet, the sunspot looks like a single pole, but it's really connected to an equal but opposite pole.

If magnetic trains are to work, wouldn't friction on the bottom of the train create thermal energy which would destroy the magnetism of the train?

When a magnetically levitated train is operating properly, it doesn't touch the track and experiences no friction. In principle, it shouldn't get hot at all. The magnetic drag effect will warm the track slightly, but that won't matter to the train's magnets. Actually, the train's magnets will almost certainly be superconducting wire coils with currents flowing in them. That type of magnet doesn't depend on the magnetic order of permanent magnets. It's the magnetic order of permanent magnets that is destroyed by heat. An electromagnetic coil will stay magnetic as long as current flows through it, even if it's so hot that it's ready to melt.

How does magnetism play a part in tapes to create sound?

The tape recorder first represents sound (pressure fluctuations in the air) as electric current and it then represents that current as magnetization of a tape. It magnetizes the tape to various depths to represent the different amounts of current and it uses the direction of the magnetization to represent which way the current should flow. During playback, the tape recorder measures just how deeply and in what direction the tape has been magnetized and uses that information to recreate the current and the sound.

Magnets can be demagnetized by heat-is that true for permanent magnets or materials that have been magnetized?

It's true for both because permanent magnets are just a special material that has been magnetized. In fact, permanent magnets are often demagnetized more easily than other simpler materials. Anything that spoils the internal order of a material (heat or vibration) can demagnetize it.

Is magnetic flux another name for magnetic field?

The two quantities are related but they're not the same. If you think of a large magnet as made up of many tiny magnets all turned in the same direction, you can think of magnetic flux as strings that connect each tiny north pole to each tiny south pole. The large magnet effectively has many of the strings extending outward from its north pole and wrapping around to its south pole. The magnetic field at each location in space around the magnet is related to how many of these strings of magnetic flux pass through a small surface at that location. Near the poles of the magnet, the density of magnetic flux lines is high and so is the magnetic field. Far from the magnet, the density of magnetic flux lines is low and the magnetic field is weak.