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Ohm's Law is simple. E = I x R or Volts = Amps x Ohms (using the proper units in the equation).
For power, P = EI, or using a substitution from Ohm's Law, P = I2R
What is the complicating factor in your original question is using 'ideal' values in a real-world application.
An alkaline cell is nominally 1.5 Volts when brand new and when the load is small (the type and size and hence capacity of the cell determines to a large extent just how small 'small' must be).
But in the real world, there is voltage sag under load and dropping voltage as the cell is used. Read through the Electronics section of CPF, especially the stickies about cell testing and all those great plots and curves and tests that SilverFox did/does on a whole bunch of different cells.
Under a 2.5 Amp load, a D-cell alkaline is -NOT- going to be supplying 1.5 volts. Nope, no way. Most brand-new never-been-used D-cell alkalines would barely be supplying 1.1 to 1.2 volts after less than 5 minutes use at a 2.5 amp rate.
Next real-world complication is that many devices are not necessarily linear with regards to power requirements. In an idealized device/world, if the power needed was constant then as the voltage supplied to the device dropped then the current needed would increase in a linear manner (from P=EI, if P=constant then changing either voltage or current means a corresponding change in the other term to maintain the constant power). In the real world, as the voltage drops the current drawn by the device may increase, decrease, or stay the same.
If your question is really "I have a portable DVD player that uses 10xD alkalines. Can I use 10xD NiMH cells instead?" my answer would be "Probably yes".
BTW, I think this -probably- should have gone in the Electronics section of CPF.
