In the modern day, chain mail is used to protect people from electrical currents. High voltage electrical workers may wear chain mail, and there's a video of a man being struck by artificial lightning while wearing chain mail. Even over long periods of time, the man does not seem to be injured, and the chain mail does not seem to be appreciably heated.
With plate armor, the conductive surfaces are larger, and thus tend to be more conductive. Furthermore, there are less jumps between armored portions that could cause problems.
Gaps between armor is something that is designed against, even in the relevant time period. Any gaps are holes that an enemy can wedge open with a sword. Furthermore, people wear leather between metal armor and their skin, which is an additional layer of insulation.
According to Wikipedia, steel structures can function as part of lightning rods.
Lightning will preferentially flow through the armor, and where it is unable to, might flow through the leather. It is extremely unlikely it will flow completely through the leather, into the body, and then back out through the leather.
Steel is a relatively good protection against lightning. For instance, it is recommended to shelter inside, for example, a car during a lightning storm. Lightning does not cause the car's body to melt, and it is even less likely to melt the less resistive (because of the higher cross-section area) steel armor.
If they are touching each other, the resistivity will further decrease, thus reducing the effect of lightning.
The thunder is around 120 decibels, equivalent to standing at the front of a rock concert for less than a second. Deafening is possible, but unlikely.
Lightning is brighter than the sun, but only for a few milliseconds. This may be enough to temporarily blind someone looking at it (and it is certain to blind someone should the lightning strike directly in the eye). However, there seems to be a dearth of information on the typical brightness of a natural lightning strike, so it is difficult to determine how long a person may be blinded. However, lack of eyesight is likely to occur for at least an eighth of a second (blink time), and it is more likely for temporary blindness to occur should the lightning strike at night – though it is essential to note that the troops will likely be more scattered at night.
In summary, lightning will likely be more effective as a psychological weapon or as a tool to frighten horses than as a direct damage-dealer. Injuries from electricity and secondary heating effects are likely to be minimal to none, injuries from sound are possible but short-lived, and injuries from light are likely, but hard to quantify.