With the prevalence of smartphones and earbuds, an app could be created that would administer a hearing test and then let the phone serve as a hearing aid for the user. The test could be as short or long as desired using test tones in various frequency ranges where the person being tested simply taps the phone when they hear the tone or stop hearing the tone. "Normal" human hearing is reportedly between 20 hz and 20 khz. Assuming the earbuds had a similar range (most don't go down to 20 hz) you could start with a muted test tone at the 20 hz frequency then gradually increase the volume until the person being tested could hear it and tap the screen. Then you could set that same test tone at a higher volume and reduce the volume until the tester could no longer hear it and they'd then tap the screen. Both volume levels should be similar, if not identical and indicate the ability of the listener to hear that test tone at a specific volume.
Depending on the thoroughness you want to use, you could retest them at every 10 hz to 100 hz and discover how well, or not, they hear at each frequency. Once you know how their hearing is, it's relatively easy to use a digital graphics equalizer to adjust the output of the source (microphones on the earbuds in this case) to the appropriate volume level for each frequency range tested. Once that was completed a simple replaying of the test tones as they've now been adjusted should have the volume sound consistent across every frequency. If it's not, a retest and redo could be done.
You could also offer different settings for different situations where sounds in specific ranges could be blocked completely, or certain sounds amplified more than necessary to make them stand out.
From a hardware standpoint (smartphone and earbuds) you could put together a package for less than $100 that would function well for most users. Software development costs would be significant but not mind-boggling. Each night the user simply removes the earbuds and smartphone and recharges them and they're good to go again the next day.
Where you get into the messy stuff is the federal, state, and local regulatory issues. Some states/areas require hearing tests to only be administered by a licensed hearing specialist. Audiologists would scream bloody murder about such a set-up as they'd be largely put out of business. Why pay thousands of dollars for a hearing aid when your smartphone and a pair of earbuds can do the job? Anyone attempting to create and market such a set-up would likely find themselves facing fines, arrest, and imprisonment unless the laws were changed.
As software goes, it's not rocket science to create it. Digital files of the test tones are readily available. Simply create a routine where each tone is played, first from a zero volume gradually ramping up until the test subject tapped the screen. Then from a volume a bit above that where the subject first tapped the phone with the volume decreasing until the person could no longer hear it and they tapped the screen again. If the two numbers are close, you move on to the next frequency. If they're not you retest once more and average the results. Once the testing portion is complete, you use a digital graphic equalizer to boost the volume of some frequencies as needed and have the user verify that the volume remains the same throughout the range. Having retest options and situational options available on the phone makes sense also. If you're going to be eating out in a crowded restaurant you can set it to mute other sounds but pass through sounds in the vocal range. None of this is hard to create. None of it should stress even the cheaper smartphones. The hardware is there and ready to use in this manner, it's just a question of someone developing the software and avoiding the regulatory headaches. (Especially the regulatory headaches.)
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