user69682: I have a DFPlayer Mini bought from AliExpress.
The player works correctly in standalone mode, but I am having troubles while using it with the following components:

Seeduino v4.0;
2GB Smartbuy MicroSD card.

When I try to use this standard sketch for Arduino to connect my Seeeduino V4 to the player:

if I set the serial like mySoftwareSerial(11, 10), the player's tiny LED turns on for 2 seconds, then it switches off, and after that, the board tells me the error message below (video here);
if I set the serial like mySoftwareSerial(10, 11), the player`s tiny LED switches on and remains being switched on, while the board tells me the error message below (video here):

DFRobot DFPlayer Mini Demo
Initializing DFPlayer ... (May take 3~5 seconds)
Unable to begin:
1.Please recheck the connection!
2.Please insert the SD card!

This module's image:

The circuit diagram:

simulate this circuit – Schematic created using CircuitLab
I have tried almost everything I could. I changed the filenames on my SD card, tried to use different pins, tried to use different wires..! I even tried to disconnect the speaker from the player!
I also had seen this message on the manufacturer`s forum:

I found that the solder joint on the TX pin was bridged to the SD card holder, effectively bridging the pin to the ground. Try a continuity test between the TX and ground. If they are connected you have a bad joint.
I was able to remove the excess solder and it worked fine after that. It's a medium difficulty soldering task.

Maybe, it`s my case?

user69682: Please check if the RX/TX pins are messed up. In my case, simply RX and TX wires' places' switching did help!

user79794: I simply removed the test for the return value of the constructor and assumed it initialised correctly.  It then ran properly every time I started it.  I have no explanation for this, but it worked.

69682: <p>I have a <a href="https://aliexpress.com/item/32657798948.html?cv=815660&amp;af=701906&amp;utm_campaign=701906&amp;aff_platform=link-c-tool&amp;utm_medium=cpa&amp;afref=https%3A%2F%2Ftrade.aliexpress.ru%2ForderList.htm%3Fspm%3Da2g0o.home.1000001.13.75df5c98WvdGQj%26tracelog%3Dws_topbar%26tsp%3D1607520665047&amp;sk=mr6wR6CC&amp;aff_trace_key=c2657ff46a764496af298ef45bc120e6-1607520697616-05778-mr6wR6CC&amp;dp=a66fcc28efcfbf5dab07e08864c73497&amp;terminal_id=049661e6cfa5453b8ac1fb4fa188d0b9&amp;utm_source=admitad&amp;utm_content=815660&amp;sku_id=59838362104" rel="nofollow noreferrer">DFPlayer Mini</a> bought from AliExpress.
The player works correctly in standalone mode, but I am having troubles while using it with the following components:</p>
<ul>
<li>Seeduino v4.0;</li>
<li>2GB Smartbuy MicroSD card.</li>
</ul>
<p>When I try to use <a href="https://wiki.dfrobot.com/DFPlayer_Mini_SKU_DFR0299#target_7" rel="nofollow noreferrer">this</a> standard sketch for Arduino to connect my Seeeduino V4 to the player:</p>
<ul>
<li>if I set the serial like <code>mySoftwareSerial(11, 10)</code>, the player's tiny LED turns on for 2 seconds, then it switches off, and after that, the board tells me the error message below (<a href="https://youtu.be/-WJ0eYgYyD8" rel="nofollow noreferrer">video here</a>);</li>
<li>if I set the serial like <code>mySoftwareSerial(10, 11)</code>, the player`s tiny LED switches on and remains being switched on, while the board tells me the error message below (<a href="https://youtu.be/a1FzlGiyJK8" rel="nofollow noreferrer">video here</a>):</li>
</ul>
<pre><code>DFRobot DFPlayer Mini Demo
Initializing DFPlayer ... (May take 3~5 seconds)
Unable to begin:
1.Please recheck the connection!
2.Please insert the SD card!
</code></pre>
<p>This module's image:
<a href="https://i.stack.imgur.com/Irdwr.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Irdwr.jpg" alt="Image" /></a></p>
<p>The circuit diagram:</p>

<p><img src="https://i.stack.imgur.com/jkQ6J.png" alt="schematic" /></p>

<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fjkQ6J.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="nofollow">CircuitLab</a></sup></p>
<p>I have tried almost everything I could. I changed the filenames on my SD card, tried to use different pins, tried to use different wires..! I even tried to disconnect the speaker from the player!</p>
<p>I also had seen <a href="https://www.dfrobot.com/forum/viewtopic.php?p=43180#p43180" rel="nofollow noreferrer">this</a> message on the manufacturer`s forum:</p>
<blockquote>
<p>I found that the solder joint on the TX pin was bridged to the SD card holder, effectively bridging the pin to the ground. Try a continuity test between the TX and ground. If they are connected you have a bad joint.
I was able to remove the excess solder and it worked fine after that. It's a medium difficulty soldering task.</p>
</blockquote>
<p>Maybe, it`s my case?</p>
 69682: <p>Please check if the <code>RX</code>/<code>TX</code> pins are messed up. In my case, simply <code>RX</code> and <code>TX</code> wires' places' switching did help!</p>
 79794: <p>I simply removed the test for the return value of the constructor and assumed it initialised correctly.  It then ran properly every time I started it.  I have no explanation for this, but it worked.</p>
