Can you connect standard speaker wire to headphone jack for example to play a portable MP3 Players' music via standard in house speakers?

Yes, you can, though the volume may not be sufficent.

If you still wish to do it, follow this guide:

1. Using either a spare audio cable or a cable bought from radio shack, use wire strippers to first cut the wire about halfway through if you have a longer cable, but if you have a short one, cut of as close to one end as you can.
2. Next, strip of about 1-2 inches of the plastic insulation. If there is a small "rip-cord"- a small piece of stranded wire or cotton used to protect the cord from being ripped in half- cut it, we won't be using it.
3. You should now have 2 wires, also, if there is a material like tin foil inside, you can cut it with scissors, its just a bit more insulation. If you have 3 wires, or if the wires are very thin, you should get a higher quality audio cable that has reasonably thick wires inside.
4. Next, If you are going to make this a permanent installation- you won't be using the speakers for anything else-, slip some heat-shrink tubing over the cord you cut and stripped. If you are going to be using the speakers for other uses, still put the heat shrink over the wires, but go to step 5 B. Otherwise, continue to step 5 A.
5. A. Using a nine volt battery, check the polarity of the speaker wires, or follow the wires to and from your speakers and mark each wire with a red and a black wire to denote negative and positive. Solder the respective wires to each other, go to step 6.
B. You will need to be able to solder for this. Using quick disconnects,
solder a 1 female connector to each wire from the speaker. Solder male connectors to the audio cable you stripped and solder male connectors to whatever wires you connected the speaker with before.
6. If you do not have a heat gun, you can improvise and use a butane lighter to carefully activate the heat shrink. Make sure it covers all exposed contacts you soldered on.
7. If you have a multimeter, great, else, improvise with a 9 volt battery. Use the continuity test on the multimeter to make sure you didn't do any cold solder joints.

8. Whenever you want to use the MP3 player or whatever with the speakers, connect the cord you made. Every other time, use the connectors you soldered on.