Take a look at the Google Chromecast Audio

https://www.google.com/chromecast/speakers/

Get a Chromecast audio. 3.5 jack out, uses your iPhone or Android phone to "cast" audio to it. If you have multiple Chromecast audios throughout your house, they can sync and play throughout.

https://www.google.com/chromecast/audio/explore/?utm_source=en-ha-na-sem&utm_medium=cpc&utm_campaign=audio&gclid=Cj0KEQjwmri_BRCZpaHkuIH75_IBEiQAIG0rIVmgEzJaHaingT7GXXgHBxSp8r0BDGAiSDcOy6yfkg0aAg3v8P8HAQ

You can also use bluetooth. Hook to a receiver using RCA or jack and then pair your phone to it. Done. Easy. Cheap.

https://www.amazon.com/Etekcity-Wireless-Bluetooth-Receiver-NFC-Enabled/dp/B00KXYXXK2/ref=sr_1_sc_2

You could just use one of these if you want to plug direct.

IMG_1362 (resized).PNG

Absolutely get some ChromeCast Audio dongles!! Especially if you want to stream from Spotify or Google Music. This is by far the best option for someone who wants to use existing stereos (or multiple stereos for perfect synchronized whole-home audio). Each unit costs about $40 and connects to the back of your stereo (very small, hides out of view). It comes with a special "combo" output jack that can be connected to a Toslink (optical audio cable) or a mini stereo plug. If your stereo is older, you will then just need an old school "RCA to mini stereo" adapter from Lowe's ($5).

Now, here is where ChromeCast shines- Whole Home Audio!!!! All units can be grouped together. I have one unit called Kitchen and another called Basement. I also have assigned both of these units to a "group" called "House Party.". When I stream to "house party" the music plays on both stereos (synched perfectly as if I had a hard-wired whole home audio system). Super cool!!!!

ChromeCast Audio units are connected to your home wifi (similar to Sonos) but unlike Sonos, you won't have to take out a second mortgage for a whole-home audio system.

FYI, don't waste your time with Bluetooth or those toy Bluetooth speakers - unless you love hearing your phone ring through your speakers during a house party. Bluetooth is great for vehicles... and that's about it. At home, extended Bluetooth use will quickly drain your phone's battery, and is a lower quality audio than Chromecast (which has HD audio).
Anyway, try it out, just make sure to ask for ChromeCast AUDIO! Otherwise the pimple face kid at Best Buy will give you the ChromeCast dongle that connects to televisions. The ChromeCast Audio comes with a short yellow audio cable.
Get one now!

Oh, maybe a Bluetooth receiver is actually what you might need.
I made the assumption that you were going to be streaming music from Spotify, Google or Pandora.
I don't even mess around with MP3's anymore, I just use Spotify.

This Logitech unit will connect to your phone via Bluetooth and your stereo with a cable. Sold by Newegg (Amazon has it a little cheaper).
http://www.newegg.com/Product/Product.aspx?Item=N82E16875873003&ignorebbr=1&cm_re=bluetooth_audio_receiver_rca-_-75-873-003-_-Product

This Etekcity unit will do it, although I've never heard of the brand for $20 it's worth a try.
https://www.amazon.com/Etekcity-Wireless-Bluetooth-Receiver-NFC-Enabled/dp/B00KXYXXK2

Bluetooth 4.1 Receiver for $19 - $6 off with coupon code BOHQXOVD at checkout = $13.

https://www.amazon.com/exec/obidos/ASIN/B01JLJMF5S

I have this unit myself. It connects via RCA cables and sounds much better than the no-name cheapies (I've tried a few), with the difference being immediately noticeable even in a modest home stereo system. It claims a 50 foot range which might be a little optimistic inside a house, but it will definitely cover the largest of rooms and beyond.

With respect to all that posted about Chromecast - note this this used wifi which is not always ideal. Our house was built in 1937 and the plaster walls are murder for effective wifi spread. A Bluetooth receiver with RCA out is a much better solution.

Or just upgrade to a $400 Yamaha receiver which had Bluetooth built in and you're done! Or better yet, the mini plug-to-RCA connector someone posted. You can't do better than a direct connection!

Yes you can. With Bluetooth you can send only the media channel to the receiver. If your phone rings you still hear that only on the phone (if desired).

You also can keep the phone in your pocket while you're playing pinball.

If your device must be directly connected there is no benefit to using a phone to play your music. Fill a cd with .mp3 files and you're good for about ten hours. Fill a dvd or Blu-ray and you have days worth on one disc.

2006 Yamaha 5 to 1 100 watt receiver with no Bluetooth capability,I have an IPhone and I would love to adapt receiver to Bluetooth capable