MMS has a size limit to media. For video, it's about 1.2Mb, but I'm not sure if this also applies to audio... it might. If you're using the stock recorder, then go into the recorder's settings and see if you can change the recording quality to "Limit for MMS". It'll record at a "timed" lower bit rate but should be fine for someone to listen to.
Note... I'm on a Note 4, but since the S5 came out the same time, I'm guessing you should see the same setting in your recording app.