Look at the top of the trunk space where the rear speaker wires are. There is probably a loose or damaged wire causing a short.
Either the sound settings are messed up, or you have blown speakers. First before buying speakers, try playing with balance and fade.
By the quality of sound coming from the speakers.
Balance
1. No power to speakers. 2. Bad sound card. 3. Speaker cables plug into the wrong jacks. 4. Bad speakers.
Your computer's sound card controls the sound coming out of your speakers. When your sound card is damaged, your sound will be distorted.
You should have sound coming out already when there's a song or sounds playing. If it's not working, check if it's muted or if the speakers are broken.
No speakers should work with any card.
Most likely there is a short in the wiring. This is a somewhat common problem with the Toyota echo, what happens is cargo in the trunk bumps the back of the rear speaker, causing a wire to come lose or break off, which can cause a short and you lose sound completely. Take a good close look at the wires to the speakers in the trunk. If you don't care about sound coming from the rear speakers and you don't want to fix it, you can just cut damaged wiring and tape it off, this will restore sound to the front speakers.
Output devices are devices that present data coming from the computer. Speakers do this by taking information that is sent to it via the sound card and outputting that information via sound.
Sound quality can be improved by getting better speakers and purchasing a quality reciever.
it means you need to buy speakers
I wish i knew the answer to this also. My Tundra has done this for several years.