I have a video that uses a heartbeat sound which sounds great and powerful on my workstation (which has a subwoofer) but when played on a mobile device or even my wimpy laptop the heartbeat sound is almost entirely non-existant. Is there a way I can adjust this so it can be heard? I assume it's because the sound is mostly in the sub / low range / whatever you call it....

With bass instruments it's generally necessary to be sure there's some definition via a higher frequency. It might be 800Hz or it might be 2kHz. With bass guitars I sometimes have to cut the lows somewhat and find a definition frequency to boost a bit. But then I generally have the other instruments as a reference.

Perhaps you should us a low shelf eq to cut the frequencies covered by your subwoofer, boost the level until it's audible, then bring the low shelf back up to fill in the lows.

You need to convince yourself first. Generate some tones. Most daws can do this, if not download from YouTube. Do a loud medium and quiet level in your editor at a range of frequencies and play the results on phones, tablets and as many computers as you can. Then you can decide on your own lowest frequency that you can be pretty sure will be heard. Subwoofers are put on people's systems because the existing system has a higher than desired cutoff at the bottom. Common sense says that as you cannot guarantee everyone has them, then anything that comes through them should be treated as optional, not essential audio. I find it quite funny when I play audio tracks on my MacBook and don't realise that a song in the key of B may have been a poor choice when the bass player has a 5 string bass and the temptation to use that bottom string was too much! The octave B is feeble on my MacBook, and th bottom B is simply missing!

Subwoofer content is enhancement or special effect use only. You editor audio should reflect the typical listening environment of the consumer.

I now edit in my audio studio and I have the editor audio output set to gently remove below 100Hz in th feed to the studio monitors for exactly this reason. I also run a spectrum scope so I can keep an eye on what is in the mix.