Now my question is, can I run it in 14.4V mode and how can I do this?

I don't believe the voltage is an optional feature. The manufacturer is just being kind enough to report to you what the specs are for each of two different (common) voltages. Most car batteries are actually 14.4 volts, regardless of what the specs on your car's electrical system might say. But there are cars (economy usually) whose batteries do not run at that voltage, and instead run closer to 12 volts. This is how it was explained to me by mechanics and car stereo professionals in my area.

As for the speakers, some stores/companies still don't bother to report the RMS handling of their speakers. But many do. What they neglect to report is the RMS range of a speaker. You can see an example of the RMS range of a speaker on Crutchfield's site, as they report this spec for nearly all of the speakers they sell. If your amplifier's RMS output is within the RMS range of the speakers, then you can safely drive that speaker with your amp. A speaker with a max RMS of 50 watts should be just fine for that amplifier, regardless of whether it is running in 12- or 14.4 volt mode. If however, the speaker's minimum RMS is 50 watts, you may want to look for a new amplifier or choose a different speaker.